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The interconnectedness of adjacent freshwater and terrestrial ecosystems results in 'tangled‐webs', where the cross‐boundary exchange of materials results in energy flows that provide critical resources for ecosystem productivity. However, these energy flows can be disrupted by anthropogenic activities, resulting in modified ecosystem functioning.A ubiquitous anthropogenic activity in fresh waters is recreational angling. Five major angling activities can disrupt the structure of the 'tangled‐web' to produce an 'angled‐web' (a tangled food web that is modified by angling activities). (1) Fish stocking increases the number and species available to exploit, but the increased fish abundances can have strong top‐down effects and disrupt reciprocal prey subsidies. Following capture, fish are either (2) harvested or (3) released alive ('catch‐and‐release0'); both can both drive eco‐evolutionary changes that can modify food‐web structure. Where fish are exploited using baited hooks, (4) the release of 'ground‐bait' is used as a fish attractant; its composition of novel protein and carbohydrate sources form novel trophic links in the food web and can accelerate eutrophication. Some angling styles also require (5) riparian and in‐stream vegetation to be cleared to minimise gear fouling, potentially modifying fish spawning and nursery areas.We suggest that each of these angling impacts (and their interactions) can contribute to transforming the 'tangled web' into an 'angled‐web', with the strength and direction of the transformation dependent on angling pressure, prevailing fishery practices, regulations and management, and the extent of the interaction effects of the activities. These interactions and their impacts will vary globally given angling methods, practices and regulations differ across the world.We argue that, in this era of substantial environmental change driven by climate change, habitat loss and nutrient enrichment of fresh waters, the extent to which angling impacts food webs should not be overlooked and underappreciated. We emphasise that when freshwater angling activities are poorly managed and/or unregulated, they can strongly modify the interconnectedness between terrestrial and freshwater food webs. [ABSTRACT FROM AUTHOR]

Native to Central and Eastern Europe, the euryhaline pikeperch Sander lucioperca can acclimatize to elevated salinity levels (e.g., up to 30‰), but it remains unknown whether their invasive populations use this ability to inhabit and/or disperse through brackish waters, such as estuaries and inshore areas. To test whether invasive pikeperch show a propensity to move into areas of relatively high salinity, their spatial use and movement patterns (e.g., home range, distances moved, and movement rates) were assessed using acoustic telemetry in the upper River Thames estuary, southeast England. Analyses revealed that individual pikeperch were capable of moving relatively long distances in a short time (e.g., speeds up to 70 m min−1), with movement patterns associated more with tidal state and elevation at the water surface (both assumed to relate to changes in salinity) than diurnal changes. There were no recorded movements of any pikeperch into the more saline, downstream waters of the estuary where salinity levels were recorded to over 40‰, with the mean salinity in the most downstream area where pikeperch were detected being 1.39‰ (range of logger: 1.22–1.71). The results suggest that these pikeperch did not use high salinity waters when less saline waters were available, and thus the risk that they will use to move through high salinity areas to expand their invasive range appears low. Accordingly, efforts to minimize risks of the further dispersal of invasive pikeperch populations can focus on control and containment programmes within fresh waters. [ABSTRACT FROM AUTHOR]

Standardised terminology in science is important for clarity of interpretation and communication. In invasion science – a dynamic and rapidly evolving discipline – the proliferation of technical terminology has lacked a standardised framework for its development. The result is a convoluted and inconsistent usage of terminology, with various discrepancies in descriptions of damage and interventions. A standardised framework is therefore needed for a clear, universally applicable, and consistent terminology to promote more effective communication across researchers, stakeholders, and policymakers. Inconsistencies in terminology stem from the exponential increase in scientific publications on the patterns and processes of biological invasions authored by experts from various disciplines and countries since the 1990s, as well as publications by legislators and policymakers focusing on practical applications, regulations, and management of resources. Aligning and standardising terminology across stakeholders remains a challenge in invasion science. Here, we review and evaluate the multiple terms used in invasion science (e.g. 'non‐native', 'alien', 'invasive' or 'invader', 'exotic', 'non‐indigenous', 'naturalised', 'pest') to propose a more simplified and standardised terminology. The streamlined framework we propose and translate into 28 other languages is based on the terms (i) 'non‐native', denoting species transported beyond their natural biogeographic range, (ii) 'established non‐native', i.e. those non‐native species that have established self‐sustaining populations in their new location(s) in the wild, and (iii) 'invasive non‐native' – populations of established non‐native species that have recently spread or are spreading rapidly in their invaded range actively or passively with or without human mediation. We also highlight the importance of conceptualising 'spread' for classifying invasiveness and 'impact' for management. Finally, we propose a protocol for classifying populations based on (i) dispersal mechanism, (ii) species origin, (iii) population status, and (iv) impact. Collectively and without introducing new terminology, the framework that we present aims to facilitate effective communication and collaboration in invasion science and management of non‐native species. [ABSTRACT FROM AUTHOR]

Understanding the strength, direction and consequences of the trophic impacts of invasive fish on functionally analogous native fishes is important in invasion impact assessment and for informing management programmes. As functionally analogous native and invasive fishes can share prey resources, there is high potential for competitive interactions, which could affect trophic niche sizes and positions.The highly invasive mrigal carp Cirrhinus mrigala and native mud carp Cirrhinus molitorella now co‐exist in many freshwaters in southern China, raising concern on the consequences for mud carp. The trophic interactions of the two species and their consequences (for growth rates and trophic niche sizes as the isotopic niche, calculated using δ13C and δ15N) were tested in a pond enclosure experiment based on additive and substitutive treatments using fish in allopatric and sympatric contexts, with results compared with invaded wild communities.In the experiment, specific growth rates decreased significantly as fish abundance in treatments increased. These density‐dependent effects were observed in both species and were independent of whether the treatment was allopatric or sympatric. In allopatric treatments, isotopic niches of mud carp increased in size with fish number. Conversely, in sympatric treatments, mud carp isotopic niche size decreased with increased mrigal carp number. For mrigal carp, isotopic niche sizes decreased as fish number increased in both allopatric and sympatric treatments.In sympatric experiment treatments, the isotopic niches of mud carp and mrigal carp were always divergent. This was, however, in contrast to four invaded wild communities, where the isotopic niches of the two fishes always had some overlap.Experimental and field data suggest that although native mud carp could be impacted by mrigal carp, these impacts might only manifest when the species are at very high abundances and inter‐specific competitive interactions could be intense. These results also suggest the two species have mechanisms that reduce their inter‐specific interactions to minimise their competitive interactions, thus any ecological impacts from mrigal carp might manifest more from indirect than direct processes. [ABSTRACT FROM AUTHOR]

Anthropogenic barriers are widely known to negatively impact the spawning migrations of anadromous fishes, by delaying or preventing passage upstream. Although the impacts of barriers on emigrating post‐spawned adults are less well studied, they could potentially impact the fitness and subsequent return rates of iteroparous species. In this study, passive acoustic telemetry was used to track the emigrations of 53 twaite shad Alosa fallax in the River Severn basin in their first spawning migration a year after being tagged, giving insights into their emigration movements and the impacts of anthropogenic weirs on these movements. A. fallax began their emigrations after spending varying amounts of time and migrating various distances within the river, with late‐emigrating individuals moving fastest and most directly. Emigrations became faster and more direct the further downstream individuals were from their furthest upstream extent. Downstream passage delays at weirs increased emigration times by a median of 61%, with environmental conditions (i.e., temperature, flow, and tidal influence on river level) having little influence on downstream passage at weirs with no modifications to facilitate fish passage. As weir‐induced emigration delays are suggested to deplete energy reserves (when energy levels are already depleted post‐spawning), limit spawning opportunities (by preventing access to downstream spawning habitat), and expose individuals to increased predation risk and suboptimal conditions (e.g., high temperatures), these delays can potentially diminish the benefits of iteroparity. The evidence presented here suggests that more consideration should be given to the potential impacts of anthropogenic barriers on the emigrations of iteroparous species when assessing river connectivity or undertaking barrier mitigation. [ABSTRACT FROM AUTHOR]

Sea lamprey Petromyzon marinus is an anadromous and semelparous fish without homing behaviors. Despite being a freshwater, free‐living organism for a large part of their life cycle, its adulthood is spent as a parasite of marine vertebrates. In their native European range, while it is well‐established that sea lampreys comprise a single nearly‐panmictic population, few studies have further explored the evolutionary history of natural populations. Here, we performed the first genome‐wide characterization of sea lamprey's genetic diversity in their European natural range. The objectives were to investigate the connectivity among river basins and explore evolutionary processes mediating dispersal during the marine phase, with the sequencing of 186 individuals from 8 locations spanning the North Eastern Atlantic coast and the North Sea with double‐digest RAD‐sequencing, obtaining a total of 30,910 bi‐allelic SNPs. Population genetic analyses reinforced the existence of a single metapopulation encompassing freshwater spawning sites within the North Eastern Atlantic and the North Sea, though the prevalence of private alleles at northern latitudes suggested some limits to the species' dispersal. Seascape genomics suggested a scenario where oxygen concentration and river runoffs impose spatially varying selection across their distribution range. Exploring associations with the abundance of potential hosts further suggested that hake and cod could also impose selective pressures, although the nature of such putative biotic interactions was unresolved. Overall, the identification of adaptive seascapes in a panmictic anadromous species could contribute to conservation practices by providing information for restoration activities to mitigate local extinctions on freshwater sites. [ABSTRACT FROM AUTHOR]

Behavioural thermoregulation enables ectotherms to access habitats providing conditions within their temperature optima, especially in periods of extreme thermal conditions, through adjustments to their behaviours that provide a "whole‐body" response to temperature changes. Although freshwater fish have been detected as moving in response to temperature changes to access habitats that provide their thermal optima, there is a lack of integrative studies synthesising the extent to which this is driven by behaviour across different species and spatial scales. A quantitative global synthesis of behavioural thermoregulation in freshwater fish revealed that across 77 studies, behavioural thermoregulatory movements by fish were detected both vertically and horizontally, and from warm to cool waters and, occasionally, the converse. When fish moved from warm to cooler habitats, the extent of the temperature difference between these habitats decreased with increasing latitude, with juvenile and non‐migratory fishes tolerating greater temperature differences than adult and anadromous individuals. With most studies focused on assessing movements of cold‐water salmonids during summer periods, there remains an outstanding need for work on climatically vulnerable, non‐salmonid fishes to understand how these innate thermoregulatory behaviours could facilitate population persistence in warming conditions. [ABSTRACT FROM AUTHOR]

Angling is a globally popular leisure activity. There are over 31 million anglers in Europe, many of which target species of the Cyprinidae family in lowland freshwater ecosystems using methods generally involving bait (e.g. groundbaits, seeds and pellets), with large bait inputs possible in periods of high angling activity. While these bait inputs act as novel trophic subsidies ('angling subsidy'), substantial knowledge gaps remain on their influence on freshwater food webs, including on fish trophic niche size and position.The effects of angling subsidies on the trophic ecology of cyprinid fish populations and their macroinvertebrate prey resources were investigated in field studies comparing waters of high angling activity ('subsidised fisheries') versus low angling activity ('non‐subsidised fisheries'), and complemented by a pond experiment using two cyprinid species in subsidy absence/presence. Methods were based on stable isotope analysis, with angling subsidies being δ13C enriched and, generally, δ15N depleted compared to macroinvertebrate prey resources.In the subsidised fisheries, while there were minimal influences of the baits on macroinvertebrate stable isotope values, the effects of the subsidies on all fish species were to substantially δ13C enrich and δ15N deplete their isotopic niches. However, patterns of interspecific niche divergence remained similar between the species in subsidy presence.In the pond experiment, there was strong isotopic association between the two fish species and macroinvertebrate putative prey in subsidy absence. In treatments that then exposed both species to angling subsidies, their stable isotope values shifted to enriched δ13C.Synthesis and application. Where angling activity is high, angling baits can provide strong trophic subsidies to freshwater fish, but with minimal effects on other trophic levels. Their regular input into freshwaters can provide some substantial benefits for fish (e.g. increased growth rates) and fisheries (e.g. elevated carrying capacity, higher catch rates), but can also increase nutrient enrichment and potentially raise concerns on angling ethics. Thus, in allowing the use of these baits, especially in relatively high quantities, managers must balance the benefits they can deliver to fish and fisheries versus the adverse effects their use can have on freshwater organisms and ecosystem functioning. [ABSTRACT FROM AUTHOR]

Populations of Atlantic salmon Salmo salar have experienced precipitous declines in abundance since the 1970s. This decline has been associated with reduced numbers of adult salmon returning to fresh water from their marine migration, i.e., their marine return rates (MRR). Thus, understanding the factors that affect MRR is of crucial conservation importance. The authors used a state‐space model with a 13‐year time series of individually tagged salmon mark‐recapture histories on the River Frome, southern England, to test the effect of smolt body length on their MRR. In addition to smolt length, the model tested for the influence of environmental covariates that were representative of the conditions experienced by the smolts in the early stages of their seaward migration, i.e., from the lower river to the estuary exit. The model indicated that, even when accounting for environmental covariates, smolt body length was an important predictor of MRR. Although larger smolts have a higher probability of returning to their natal river as adults than smaller smolts, and one‐sea‐winter salmon have a survival rate twice as high as multi‐sea‐winter salmon, the actual biological mechanisms underpinning this phenomenon remain uncertain. These results have important applications for salmon conservation, as efforts to bolster salmon populations in the freshwater environment should consider methods to improve smolt quality (i.e., body size) as well as smolt quantity. [ABSTRACT FROM AUTHOR]

Anthropogenic activities are increasingly threatening aquatic biodiversity, especially anadromous species. Monitoring and conservation measures are thus required to protect, maintain and restore imperilled populations. While many species can be surveyed using traditional capture and visual census techniques, species that use riverine habitats in a less conspicuous manner, such as sea lamprey Petromyzon marinus, can be more challenging to monitor. Sea lamprey larvae (ammocoetes) can spend several years in freshwater burrowed within soft sediments, inhibiting their detection and assessment. Here, we present a qPCR assay based on the detection of environmental DNA (eDNA) to identify the presence of ammocoetes burrowed in the sediment. We present an extensively validated method that ensured both species‐specificity of the assay as well as the capacity to detect ammocoetes when abundances are low. Experiments on burrowing activity suggested that most of the DNA released into the sediment occurs during burrowing. Overall, we demonstrate this new molecular‐based tool is an efficient and effective complement to traditional monitoring activities targeting larval stages of sea lampreys. [ABSTRACT FROM AUTHOR]

Ecological studies on the critically endangered European eel Anguilla anguilla often incorporate stable isotope analysis that typically uses dorsal muscle sampled from euthanised eels. To minimise the lethal sampling of imperilled populations, fin tissue and/or epidermal mucus can provide non‐lethal alternatives to muscle. The results here indicate that δ13C and δ15N values of both eel fin and mucus are not significantly different from those of muscle and can be applied directly in comparative SI studies. [ABSTRACT FROM AUTHOR]

Aim: The relationships between species and their landscape are important for understanding migration patterns. In fluvial systems, the complexity of the river network can strongly influence the dispersal and colonization rates of invading alien fishes, but habitat quality, species' biological traits and their location of introduction are also potentially important. However, understandings of how these factors interact in the wild to influence the spatial distribution of invasive species over time are limited from empirical studies. Location: "Virtual" and "real‐world" rivers from England and Wales. Method: We developed an individual‐based model (IBM) to predict how these different factors influenced the invasion dynamics and population growth rates (as abundances) of nine "virtual" alien fishes over two timeframes (10 and 30 years). The alien fishes differed in their demographic (r‐ to K‐selected) and dispersal (fast to slow) characteristics and the rivers in their network complexity. Results: Irrespective of river type, species and timeframe, the main drivers of both dispersal and population growth were the location of the introduction and the mean habitat quality of the patch into which the species were released. The introduction location determined whether dispersal was mainly passive in a downstream direction (faster) or active in an upstream direction (slower), with higher habitat quality then enabling faster population growth rates. Over 30 years, invasion rates were predicted to increase as the complexity of the river network increased, as this opened multiple invasion fronts where the invader traits favoured faster dispersal. Main conclusions: This novel IBM revealed how the complexity of the physical environment interacts with the biological traits of alien species to influence invasion outcomes, with the location of the introduction and its habitat quality being the most important factors. These results thus substantially increase understanding of the factors that influence the dispersal and colonization rates of alien freshwater fishes. [ABSTRACT FROM AUTHOR]

Gordon H. Copp, a renowned fish biologist and treasurer of the Fisheries Society of the British Isles (FSBI) for 8 years, passed away at the age of 67. He had a diverse career that began in chemistry and biotechnology before transitioning to fish biology. His research focused on topics such as fish reproduction, nursery sites, population dynamics, and the ecology of non-native fish species. Copp published over 230 papers and mentored numerous early career researchers. He also played a crucial role in revolutionizing the FSBI's finances, leaving a lasting legacy in the field of fish biology. [Extracted from the article]

1. The effects of biological invasions on native biodiversity have resulted in a range of policy and management initiatives to minimize their impacts. Although management options for invasive species include eradication and population control, empirical knowledge is limited on how different management strategies affect invasion outcomes. 2. An individual-based model (IBM) was developed to predict how different removal ('culling') strategies affected the abundance and spatial distribution of a virtual, small-bodied, r-selected alien fish (based on bitterling, Rhodeus sericeus) across three types of virtual river catchments (low/intermediate/high branching tributary configurations). It was then applied to nine virtual species of varying life-history traits (r- to K-selected) and dispersal abilities (slow/intermediate/fast) to identify trade-offs between the management effort applied in the strategies (as culling rate and the number of patches it was applied to) and their predicted effects. It was also applied to a real-world example, bitterling in the River Great Ouse, England. 3. The IBM predicted that removal efforts were more effective when applied to recently colonized patches. Increasing the cull rate (proportion of individuals removed per patch), and its spatial extent was effective at controlling the invasive population; when both were relatively high, population eradication was predicted. 4. The characteristics of the nine virtual species were the main source of variation in their predicted abundance and spatial distribution. No species were eradicated at cull rates below 70%. Eradication at higher cull rates depended on dispersal ability; slow dispersers required lower rates than fast dispersers, and the latter rapidly recolonized at low cull rates. The trade-offs between management effort and the outcomes of the invasion were, generally, optimal when intermediate effort was applied to intermediate numbers of patches. In the Great Ouse, model predictions were that management interventions could restrict bitterling distribution by 2045 to 21% of the catchment (versus 90% occupancy without management). 5. Synthesis and application. This IBM predicted how management efforts can be optimized against invasive fishes, providing a strong complement to risk assessments. We demonstrated that for a range of species' characteristics, culling can control and even eradicate invasive fish, but only if consistent and relatively high effort is applied. [ABSTRACT FROM AUTHOR]

Novel trophic interactions between invasive and native species potentially increase levels of interspecific competition in the receiving environment. However, theory on the trophic impacts of invasive fauna on native competitors is ambiguous, as while increased interspecific competition can result in the species having constricted and diverged trophic niches, the species might instead increase their niche sizes, especially in omnivorous species.The competitive interactions between an omnivorous invasive fish, common carp Cyprinus carpio, and a tropically analogous native and threatened fish, crucian carp Carassius carassius, were tested using comparative functional responses (CFRs). A natural pond experiment then presented the species in allopatry and sympatry, determining the changes in their trophic (isotopic) niche sizes and positions over 4 years. These predictive approaches were complemented by assessing their trophic relationships in wild populations.Comparative functional responses revealed that compared to crucian carp, carp had a significantly higher maximum consumption rate. Coupled with a previous cohabitation growth study, these results predicted that competition between the species is asymmetric, with carp the superior competitor.The pond experiment used stable isotope metrics to quantify shifts in the trophic (isotopic) niche sizes of the fishes. In allopatry, the isotopic niches of the two species were similar sized and diverged. Conversely, in sympatry, carp isotopic niches were always considerably larger than those of crucian carp and were strongly partitioned. Sympatric crucian carp had larger isotopic niches than allopatric conspecifics, a likely response to asymmetric competition from carp. However, carp isotopic niches were also larger in sympatry than allopatry. In the wild populations, the carp isotopic niches were always larger than crucian carp niches, and were highly divergent.The superior competitive abilities of carp predicted in aquaria experiments were considered to be a process involved in sympatric crucian carp having larger isotopic niches than in allopatry. However, as sympatric carp also had larger niches than in allopatry, this suggests other ecological processes were also likely to be involved, such as those relating to fish prey resources. These results highlight the inherent complexity in determining how omnivorous invasive species integrate into food webs and alter their structure. [ABSTRACT FROM AUTHOR]

River ecosystems are often fragmented by artificial structures, such as weirs. For anadromous species, these structures can impede access to upstream spawning sites and ultimately lead to severe population declines.This study focused on the freshwater spawning migration of the sea lamprey, Petromyzon marinus, an anadromous species threatened by habitat fragmentation across its native range. To quantify the cumulative impacts of multiple weirs on upstream‐migrating adults, and to explore the environmental factors affecting migratory movements, passive acoustic telemetry was applied to 56 individuals during their spawning migration in the heavily fragmented River Severn basin, UK.While 89% of tagged sea lamprey passed the first weir upstream of the release site on the main river, only 4% passed the fifth weir. For 85% of migrants, the upstream extent of migration was immediately downstream of a weir. Individuals that passed weirs upstream of the release site (n = 50) took 21.6 ± 2.8 days to reach their most upstream location, experiencing cumulative passage times at weirs of 15.7 ± 2.8 days; these delays constituted a median of 84% of total upstream movement times.Multistate models showed that the weir passage rates of sea lamprey in tidal and non‐tidal areas increased significantly when downstream river level and discharge were elevated. Upstream‐to‐downstream changes in direction were frequent downstream of weirs, but rare in unobstructed river sections.The results provided evidence for a cumulative effect of multiple weirs on sea lamprey movements, substantially delaying upstream migrants and limiting their spawning to atypical habitat. The results also demonstrated the crucial roles of high tides and elevated discharge events in enabling weir passage. Although the Severn Estuary features conservation designations for sea lamprey, this study reveals that barriers are inhibiting their upstream migration, a problem that should be addressed to assist sea lamprey conservation. [ABSTRACT FROM AUTHOR]

Migration enables animals to access important resources throughout their lifetime but exists in a trade‐off with elevated mortality risk. In spring, juvenile Atlantic salmon (smolts) migrate from their natal rivers for marine feeding grounds, with the timing of their marine entry a potentially important determinant of their long‐term survival. However, there is relatively little known on how the interaction of biological and environmental factors affect smolt migration phenology at the individual level, and how these vary throughout the duration of the smolt seaward migration (run).Using 15‐year tag, recapture, and detection datasets of individual smolts (marked with passive integrated transponder tags) from a chalk stream in southern England, the influences of a range of biological and environmental variables were tested on the run timing of individual smolts, measured as the timing of their arrival in a lower river reach.The probability of smolts arriving earlier in the lower river reach was elevated following winters that were relatively warm, and when there were larger positive daily changes in water temperature and discharge during the run. Early migrants tended to be larger individuals and from sites lower in the catchment, from where the smolts had to migrate relatively shorter distances. Later migrants were more likely to migrate in schools, but with schooling behaviour also more likely to occur during daylight than at night.The relative influence of some of these variables altered throughout the run. Relative changes in daily water temperature were not important during the middle period of the smolt run but were important at the start and end of the run. Relative changes in daily discharge were most influential towards the end of the run, when even relatively small changes in discharge had a strong influence on migration.These results reveal the importance of a wide range of biological and environmental variables on the phenology of smolt migrations, and how their influence can alter throughout the run. With predictions of annually increasing river temperatures, more frequent and intense discharge events, and associated shifts to earlier migration, these results emphasise that such changes in climate are likely to have substantial consequences on the future success of smolt migrations and thereby future numbers of returning adult spawners. [ABSTRACT FROM AUTHOR]

Human‐induced environmental change is a major stressor on freshwater habitats that has resulted in the population declines of many freshwater species. Ontogenetic shifts in habitat use and associated (st)age‐specific requirements mean that impacts of environmental stressors can influence (st)ages in a population differently, and yet relatively few studies of freshwater fish populations account for their detail.We aimed to identify environmental and biotic factors affecting survival estimated for six age‐classes of a European grayling population in the River Wylye, UK over a 17‐year period. We used a Bayesian age‐structured state space model to estimate survival of grayling cohorts between subsequent life stages (eggs to age 5 adults) for 16 annual transitions (2003–2004 to 2018–2019), whilst accounting for imperfect sampling of the population. We quantified the effects of seasonal water flow and temperature, in‐stream habitat and prey resource, and potential competitors and predators on survival between subsequent life stages. We used Bayesian variable selection to gauge their relative importance on survival.Grayling abundances declined during the study period (>75% in all age‐classes), predominately driven by a loss of mature adults. Changes to seasonal flows negatively influenced their survival: increased days of summer low flow related to decreased survival of subadults and mature adults, and lower winter flows related to reduced recruitment of juveniles from eggs. Higher summer macrophyte cover negatively influenced juvenile and subadult survival and increasing days of high temperature in summer appeared detrimental to juvenile survival. Abundance of brown trout (a potential competitor and predator) did not negatively influence grayling survival.Our results reveal the implications of environmental change on a salmonid population, where recent low summer flows and high temperatures, and below average winter flows, have negatively influenced grayling survival. These conditions appear to be becoming more frequent and persistent in our study river, which is towards the species' southern range limit, which could render the population vulnerable to climate change. Our study demonstrates how careful analysis of long‐term population monitoring and environmental datasets can identify factors affecting (st)age‐specific fish population dynamics, and when combined with local expertise, results in realistic mitigation proposals to promote wildlife population persistence. [ABSTRACT FROM AUTHOR]

Artificial barriers on lowland rivers impede the spawning migrations of anadromous fishes, preventing access to historical spawning areas. In the cryptic European shads Alosa alosa and Alosa fallax ('shad' hereafter), this has resulted in population declines across their range. Conservation programmes aim to facilitate the passage of migrators over these barriers and so require baseline information on the spatial and temporal extent of current migrations.Here, a shad‐specific environmental DNA (eDNA) assay was used to quantify the spatial extent of shad spawning migrations in the River Severn basin, western England. This basin is characterized by the presence of multiple barriers in the lower catchment. In 2017, the eDNA assay was piloted in the River Teme, an important shad spawning tributary, and then applied in 2018 and 2019 across the lower Severn basin.In all years, shad DNA was detected between mid‐May and mid‐June, with the maximum spatial extent of shad distribution being in early June when shad eDNA was detected upstream of weirs that were generally considered as impassable. In 2018, this included the detection of shad above the most upstream weir on the main River Severn that required individual fish to have passed six weirs.Although barriers inhibit the spawning migrations of shad, this eDNA assay showed that some highly vagile individuals might be able to ascend these barriers and migrate considerable distances upstream. This suggests that efforts to increase the permeability of these barriers could result in relatively high numbers of migrating shad reaching upstream spawning areas. These results demonstrate that this eDNA assay could also be used across their range, to further quantify the spatial extent of their spawning, including in highly fragmented rivers and those where shad are believed to spawn only occasionally and are rarely observed. [ABSTRACT FROM AUTHOR]

Studies suggest the migratory behaviours of potamodromous fishes can be highly variable in barrier‐free systems, where differing movement types enable populations to exploit a wide range of food and space resources. This intra‐population diversity in spatial and temporal resource use is important to our ecological understanding of fish distribution patterns and population structure. Despite this, freshwater ecosystems are increasingly characterised by high levels of fragmentation and degradation that restrict mobile fauna, and limit opportunities to study natural, unconstrained movement behaviour.Common bream Abramis brama (bream) is a potentially strong model species for testing the importance of diverse migration patterns in lowland rivers, but existing studies have been largely restricted to spatially confined and/or anthropogenically modified systems. This study's principal focus was to examine the diversity of bream movement behaviour in a highly connected, lowland system using passive acoustic telemetry, which provided continuous, multi‐year data on the movements of 181 bream across a tidally influenced, lowland wetland in eastern England (c. 60 km of continuous river length plus numerous interconnected shallow lakes and dykes). Tracked bream were grouped according to their initial location and timing of tagging.Bream migratory behaviours varied considerably between tagging groups, but with greater consistency within groups. There was little mixing of groups outside of spawning periods, with season and tidal phase being significant predictors of movement. Rates of movement and swimming speeds were highest in spring, with movements also generally occurring in the direction of tidal flows.For fish sampled just prior to spawning, there was considerable diversity in their post‐spawning behaviour, with some remaining in the immediate vicinity of the sampling location and others that moved to areas c. 25 km away. These spatially discrete patterns remained until the following spawning period.These results suggest that this lowland fish population is comprised of several distinct, semi‐independent subpopulations that only share space resources in their spawning period. This indicates the importance of connectivity in lowland freshwater systems for enabling and maintaining high phenotypic diversity in the movement behaviours of potamodromous fishes. [ABSTRACT FROM AUTHOR]

Microplastics (MPs) are small, plastic particles of various shapes, sizes and polymers. Although well studied in marine systems, their roles and importance in freshwater environments remain uncertain. Nevertheless, the restricted ranges and variable traits of freshwater fishes result in their communities being important receptors and strong bioindicators of MP pollution. Here, the current knowledge on MPs in freshwater fishes is synthesized, along with the development of recommendations for future research and sample processing. MPs are commonly ingested and passively taken up by numerous freshwater fishes, with ingestion patterns often related to individual traits (e.g. body size, trophic level) and environmental factors (e.g. local urbanization, habitat features). Controlled MP exposure studies highlight various effects on fish physiology, biochemistry and behaviour that are often complex, unpredictable, species‐specific and nonlinear in respect of dose–response relationships. Egestion is typically rapid and effective, although particles of a particular shape and/or size may remain, or translocate across the intestinal wall to other organs via the blood. Regarding future studies, there is a need to understand the interactions of MP pollution with other anthropogenic stressors (e.g. warming, eutrophication), with a concomitant requirement to increase the complexity of studies to enable impact assessment at population, community and ecosystem levels, and to determine whether there are consequences for processes, such as parasite transmission, where MPs could vector parasites or increase infection susceptibility. This knowledge will determine the extent to which MP pollution can be considered a major anthropogenic stressor of freshwaters in this era of global environmental change. [ABSTRACT FROM AUTHOR]

Assessments of patterns of animal movements are important for understanding their spatial ecology. Geostatistical models of stable isotope (SI) landscapes (isoscapes) provide a complementary tool to telemetry for assessing and predicting animal movements, but are rarely applied to riverine species. Often single‐isotope gradients in freshwater environments are insufficiently variable to provide high isoscape resolution at relatively fine spatial scales. This is potentially overcome using dual‐isotope assignment procedures, and thus, the aim here was to apply single (δ13C) and dual (δ13C and δ15N) isoscapes to assigning riverine fish to origin and predicting their movements. Using the River Bure, England, as the study system, the foraging locations of a small‐bodied lowland river fish (roach Rutilus rutilus) of low vagility were predicted using their SI data and those of a common prey item (amphipods). These foraging locations were then compared to their capture locations, with the distance between these being their predicted displacement distance. The results indicated significant enrichment of δ13C and δ15N with distance downstream in roach fin tissue and amphipods; roach bivariate isotopic niches were spatially variable, with no niche overlap between upstream and downstream river reaches. Furthermore, the dual‐isoscape assignment procedure resulted in the lowest predicted displacement distances for roach, therefore enhancing model performance. The dual‐isoscape approach was then applied to determining the predicted displacement distance of individual common bream Abramis brama, a larger, more vagile species, with these data then compared against the subsequent spatial extent of their movements recorded by acoustic telemetry. When using a high probability density threshold for isotope assignment, the predicted displacement distance of common bream was a significant predictor of the spatial extent of their subsequent movements recorded by acoustic telemetry, although it was less able to predict the direction of displacement. This first probabilistic assignment to origin for riverine species using a dual‐isotope isoscape technique demonstrated that where the required spatial resolution of animal movements in freshwater is moderately broad (5–10 km), dual‐isotope isoscapes can provide a reliable alternative or complementary method to telemetry. [ABSTRACT FROM AUTHOR]

Life history plasticity can be a strong predictor of the establishment and invasion success of introduced fishes. Here, the life‐history traits of four P. parva populations in adjacent ponds in Southern England were measured throughout 2013 to determine the timing and length of their reproductive season and the extent of trait plasticity. The relative abundance of the populations (as catch per unit effort) was similar, with low variability in their traits relating to reproductive effort and somatic growth. All the populations were male dominated. Both sexes matured at small body sizes, with fish as small as 30 mm being mature in both sexes, with the age at maturity for both sexes being age 1+ years. The peak spawning period, characterised by female fish investing heavily in reproduction when their gonado‐somatic index (GSI) values were highest and declined thereafter, occurred in May and June, and after a mean of 212 ± 24 degree‐days >12°C, and with water temperatures being 13.2–14.6°C. There were no further peaks in GSI, despite the species being considered to have prolonged spawning periods across spring and summer. These results also suggest that whilst the expression of life‐history traits of invasive fishes often vary with density, they can be highly consistent between populations at similar abundances and locations. [ABSTRACT FROM AUTHOR]

Determining the comparative impacts of increased intra‐ versus inter‐specific competition is important in freshwater ecosystems for understanding the ecological changes that can result from activities such as fish stocking events (using alien and/or native fish species), as well as from natural processes that elevate population abundances (e.g. increased annual recruitment success). While increased inter‐specific competition can result in slower growth rates and/or reduced population density in the weaker or less abundant competitor, it is important that this is assessed in relation to the impacts of increased intra‐specific competition.We tested how the strength of inter‐specific competition from a co‐existing species varies with abundance, and how this compares with increased intra‐specific competition. Fish were the model taxa, as their growth rates strongly correlate with competitive success. Replicated pond mesocosms (150 days) used chub Squalius cephalus in an allopatric control (n = 5; C5) and allopatric treatment (n = 10; C10), and in sympatric treatments (n = 5) with European barbel Barbus barbus (n = 5 (T1), 10 (T2) and 15 (T3)). Treatment effects were tested on fish‐specific growth rates (SGRs), and the size and position of the trophic and isotopic niche (stomach contents and stable isotope [SI] analyses respectively).Chub SGRs were significantly higher in C5 versus all other treatments, but did not differ among the other allopatric and sympatric treatments. Chub trophic niche sizes in T1 to T3 were significantly smaller than C5, indicating more specialised diets in the presence of barbel. Chub trophic niche size in C10 was, however, larger than C5 and T1, indicating a shift to a more generalised diet as intra‐specific competition increased.As SGRs reduced in treatments, so did the predicted extent of fish SI turnover, with SI data in T1 to T3 not at isotopic equilibrium with their diet in the mesocosms at the experiment's end. Following conversion of fish SI data to represent values at 95% isotopic turnover, chub isotopic niches also revealed shifts to a more general diet as intra‐specific competition increased, but to more specialised diets as inter‐specific competition increased.Increased intra‐ and inter‐specific competition impacts on the trophic and isotopic niches were contrasting; both metrics indicated niche constrictions in sympatry but niche expansions in allopatry. Impacts on fish growth were evident from both. These results emphasise that the trophic consequences of competition in freshwater fish can differ between stocking events involving the release of conspecifics or other species, with this having important considerations for how freshwater fish communities are managed for angling exploitation. [ABSTRACT FROM AUTHOR]

Biotelemetry is a central tool for fisheries management, with the implantation of transmitters into animals requiring refined surgical techniques that maximize retention rates and fish welfare. Even following successful surgery, long‐term post‐release survival rates can vary considerably, although knowledge is limited for many species. The aim here was to investigate the post‐tagging survival rates in the wild of two lowland river fish species, common bream Abramis brama and northern pike Esox lucius, following their intra‐peritoneal double‐tagging with acoustic transmitters and passive integrated transponder (PIT) tags. Survival over a 2‐year period was assessed using acoustic transmitter data in Cox proportional hazards models. Post‐tagging survival rates were lowest in the reproductive periods of both species, but in bream, fish tagged just prior to spawning actually had the highest subsequent survival rates. Pike survival was influenced by sex, with males generally surviving longer than females. PIT tag detections at fixed stations identified bream that remained active, despite loss of an acoustic transmitter signal. In these instances, loss of the acoustic signal occurred up to 215 days post‐tagging and only during late spring or summer, indicating a role of elevated temperature, while PIT detections occurred between 18 and 359 days after the final acoustic detections. Biotelemetry studies must thus always consider the date of tagging as a fundamental component of study designs to avoid tagged fish having premature end points within telemetry studies. [ABSTRACT FROM AUTHOR]

The mahseer (Tor spp.) fishes of South and Southeast Asia are iconic megafaunal species that are highly valued by recreational anglers. Knowledge on their populations is limited owing to the challenges associated with sampling these large‐bodied fishes (>50 kg) in remote monsoonal rivers.Despite its global iconic status among recreational anglers, the hump‐backed mahseer of the Cauvery River (South India) lacked a valid scientific name and was on a trajectory towards extinction until its rapidly declining population status was established by analyses of angler catch records.Angling records from 1998 to 2012 showed that mahseer catch rates had increased in this period. The resulting publication in Aquatic Conservation (AQC) highlighted the positive role of catch‐and‐release angling in providing information on data‐poor species. However, further analyses showed that these catches comprised not one but two distinct phenotypes.Before 1993, all mahseer captured were hump‐backed; since then, a blue‐fin phenotype appeared in catches and subsequently dominated them. These results triggered further studies indicating that the hump‐backed mahseer was the endemic Tor remadevii and that the blue‐fin was the invasive Tor khudree, introduced in 1976 and then stocked periodically from hatcheries.The initial AQC publication successfully demonstrated the high value of organized angling as a monitoring tool for data‐poor fishes and its application to assessing the temporal population patterns of large‐bodied fishes in monsoonal rivers. It was also the catalyst for initiating subsequent studies on T. remadevii that, together, enabled its recent assessment as 'Critically Endangered' on the International Union for Conservation of Nature (IUCN) Red List. In the absence of the AQC paper, and the subsequent studies that it triggered, it is highly probable that the species would have remained on a trajectory towards rapid extinction. Instead, the first major steps to safeguarding its future have been taken. [ABSTRACT FROM AUTHOR]

Individual variability in dispersal strategies, where some individuals disperse and others remain resident, is a common phenomenon across many species. Despite its important ecological consequences, the mechanisms and individual consequences of dispersal remain poorly understood. Here, riverine Northern pike (Esox lucius) juveniles (age 0+ (young‐of‐the‐year) and 1+ years) were used to investigate the influence of body size and trophic position (at capture) on the dispersal from off‐channel natal habitats and the subsequent consequences for body sizes, specific growth rate and trophic position (at recapture). Individuals that dispersed into the river ("dispersers") were not significantly different in body size than those remaining on nursery grounds ("stayers"). For trophic position, 0+ dispersers were of significantly lower trophic position than stayers, but with this not apparent in the 1+ fish. Following dispersal into the river, the dispersers grew significantly faster than stayers and, on recapture, were significantly larger, but with no significant differences in their final trophic positions. Early dispersal into the river was, therefore, not associated with early dietary shifts to piscivory and the attainment of larger body sizes of individuals whilst in their natal habitats, contrary to prediction. These results suggest that despite an increasing risk of mortality for individuals dispersing early from natal areas, there are long‐term benefits via elevated growth rates and, potentially, higher fitness. Such early dispersal behaviour could be driven by early competitive displacement. [ABSTRACT FROM AUTHOR]

Suitable gravel availability is critical for the spawning success of lithophilous fishes, including redd builders. Redd construction during spawning can alter substrate characteristics, thereby influencing hydraulic conditions and sediment transport, highlighting the importance of spawning as a zoogeomorphic activity. Here, interactions between redd‐building fish and their spawning environment were investigated for European barbel Barbus barbus with a comparative approach across three English rivers: Teme (western), Great Ouse (eastern) and Idle (central). Sediment characteristics of spawning habitats were similar across the rivers, including subsurface fine sediment (<2 mm) content (≈20% dry weight), but elevated subsurface silt content and coarser surface sediments were found in the river Teme. Water velocities were similar at spawning sites despite differences in channel width and depth. Redds were characterized by a pit and tailspill, with no differences in surface grain‐size characteristics between these and the surrounding riverbed, but with topographic alteration (dimensions and tailspill amplitude) in line with those of salmonids. Estimates of the fraction of the bed that spawning barbel were capable of moving exceeded 97% in all rivers. Estimated reproductive potential varied significantly between the rivers Idle and Teme (3,098 to 9,715 eggs/m2), which was largely due to differences in barbel lengths affecting fecundity. Larger barbel, capable of producing and depositing more eggs, but in more spatially extensive redds, meaning fewer redds per given surface area of riverbed. Predictions of barbel egg mortality based on sand content were low across both rivers. The effects of silt on barbel egg and larvae development are unknown, but the levels detected here would significantly impact salmon egg mortality. Similarities in fish length to redd area and the size of moveable grains by spawning barbel and salmon suggest they have similar geomorphic effects on sediments, although fine sediment tolerance is highly divergent. [ABSTRACT FROM AUTHOR]

Anadromous fishes can be important prey resources for piscivorous fauna in lowland rivers. Freshwater anglers exploiting large‐bodied cypriniform fishes use high quantities of pelletised marine fishmeal baits that can contribute substantially to fish diets. This marine‐derived energy pathway also potentially provides a marine prey resource for freshwater piscivores. However, large‐bodied cypriniform fishes are often in a size refuge against predation due to their large sizes.Stable isotope (δ15N and δ13C) analysis assessed how novel marine prey resources influenced the diet of a freshwater apex predator, northern pike Esox lucius, in an impounded river basin (lower River Severn, western England). Up to three groups of prey resources were present: anadromous European shad (Alosa spp.), cypriniform fishes with dietary specialisms based on marine fishmeal baits, and freshwater prey. The availability of these prey resources to E. lucius varied according to river connectivity and levels of angling exploitation in different river reaches.Where the three prey groups were present, E. lucius were more enriched in δ13C values (range: −24.74 to −16.34‰) compared to river reaches where aspects of the marine prey groups were absent (range: −28.30 to −21.47). In all reaches, δ13C increased as E. lucius length increased. In the reach where all prey groups were present, the isotopic niches of three E. lucius size categories were strongly partitioned; this was not apparent in reaches where the marine pathways were unavailable.Stable isotope mixing models suggested that freshwater prey were the most important prey item, contributing between 42% and 96% to the diet of individual E. lucius. However, where present, anadromous fishes and cypriniform fishes specialising on marine fishmeal baits were also important prey items, contributing substantially to the diet of larger E. lucius (length > 650 mm). The total dietary contributions of the marine resources varied considerably among the individual larger fish (22–58% of total diet).The presence of two marine resource pathways in a lowland river thus strongly influenced the diet of an apex predator, but with contributions being a function of their spatial availability, E. lucius body size and individual trophic specialisations. These results emphasise how the anthropogenic activities of river engineering and human subsidies can affect the trophic dynamics of apex predators. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems constitute only a small fraction of the planet's water resources, yet support much of its diversity, with freshwater fish accounting for more species than birds, mammals, amphibians or reptiles. Fresh waters are, however, particularly vulnerable to anthropogenic impacts, including habitat loss, climate and land use change, pollution and biological invasions. This environmental degradation, combined with unprecedented rates of biodiversity change, highlights the importance of robust and replicable programmes to monitor freshwater fish. Such monitoring programmes can have diverse aims, including confirming the presence of a single species (e.g., early detection of alien species), tracking changes in the abundance of threatened species, or documenting long‐term temporal changes in entire communities. Irrespective of their motivation, monitoring programmes are only fit for purpose if they have clearly articulated aims and collect data that can meet those aims. This review, therefore, highlights the importance of identifying the key aims in monitoring programmes and outlines the different methods of sampling freshwater fish that can be used to meet these aims. We emphasize that investigators must address issues around sampling design, statistical power, species' detectability, taxonomy and ethics in their monitoring programmes. Additionally, programmes must ensure that high‐quality monitoring data are properly curated and deposited in repositories that will endure. Through fostering improved practice in freshwater fish monitoring, this review aims to help programmes improve understanding of the processes that shape the Earth's freshwater ecosystems and help protect these systems in face of rapid environmental change. [ABSTRACT FROM AUTHOR]

Ecological theory on the trophic impacts of invasive fauna on native competitors is equivocal. Whilst increased interspecific competition can result in coexisting species having constricted and diverged trophic niches, the competing species might instead increase their niche sizes to maintain energy intakes. Empirical experiments can test invasion theory on competitive interactions and niche sizes across different spatial scales and complexity.The consequences of increased interspecific competition from a model alien fish Leuciscus idus were tested on two taxonomically and trophically similar native fishes, Squalius cephalus and Barbus barbus. Competitive interactions were tested in tank aquaria using comparative functional responses (CFRs) and cohabitation trials. The consequences of these competitive interactions for the trophic niche sizes and positions of the fishes were tested in pond mesocosms.Comparative functional responses revealed that compared to B. barbus, L. idus had significantly higher attack and consumption rates; cohabitation trials revealed B. barbus growth rates were depressed in sympatry with L. idus. For L. idus and S. cephalus, differences in their functional response parameters and growth rates were not significant.Pond mesocosms used stable isotope metrics to quantify shifts in the trophic niche sizes of the fishes between allopatry and sympatry using a substitutive experimental design. Isotopic niches were smaller and more divergent in sympatric paired species than predicted by their allopatric treatments, suggesting trophic impacts from interspecific competition. However, an all‐species sympatric treatment revealed similar niche sizes with allopatry. This maintenance of niche sizes in the presence of all species potentially resulted from the buffering of direct competitive effects of the species pairs by indirect effects.Experimental predictions from tank aquaria assisted the interpretation of the constricted and diverged trophic niches detected in the paired‐species sympatric treatments of the pond mesocosms. However, the all‐species sympatric treatment of this experiment revealed greater complexity in the outcomes of the competitive interactions within and between the species. These results have important implications for understanding how alien species integrate into food webs and influence the trophic relationships between native species. [ABSTRACT FROM AUTHOR]

Introduced fishes can develop invasive populations that impact native species and ecosystems. Understanding the population ecology of introduced species in their extended ranges and how this compares to their native ranges is therefore important for informing their management. Here, the age and somatic growth rates of the piscivorous freshwater fish pikeperch Sander lucioperca were analysed across their invasive and native ranges to determine their spatial patterns and drivers. Analyses were initially completed in their invaded range in central and western England. Populations varied spatially in their growth rates; being slowest for a population in a narrow and shallow canal and fastest in a large, impounded lowland river. A meta‐analysis of parameters of the von Bertalanffy growth model then revealed that across their native and invasive ranges, their theoretical ultimate lengths (L∞) and growth coefficients (K) were significantly related to latitude, but not longitude. Their relationships with latitude were nonlinear, with higher values of L∞ and lower values of K being evident towards their northerly and southerly range limits. Faster growth rates were evident in the middle of their range (45 to 55°N), suggesting temperatures here were most optimal for growth, but were in a trade‐off with reduced ultimate lengths. These spatial patterns suggest that whilst introduced S. lucioperca can colonise new waters across a wide area, the expression of their life‐history traits will vary spatially, with potential implications for how invasive populations establish and integrate into native fish communities. [ABSTRACT FROM AUTHOR]

The spawning success of lithophilic salmonids is strongly influenced by the fine sediment content ("fines") of spawning substrates, yet knowledge on the impacts of fines on the spawning of non‐salmonid lithophiles remains limited, despite their ecological and socio‐economic importance in European rivers. Consequently, the aim here was to use an ex‐situ experiment to investigate the impact of sand content on egg survival and timing of larval emergence of the surface‐spawning cyprinid European barbel Barbus barbus. Thirty incubator boxes within a recirculating system were filled with one of five experimental sediment mixtures (0%–40% sand by mass) that each contained 300 fertilised eggs at a depth of 50 mm. Emerged, free‐swimming larvae were captured and counted daily to assess grain‐size effects on larval survival and emergence. Specifically, total proportion of emerged larvae, cumulative daily proportion of emerged larvae and time required to reach 50% emergence were measured during the study. Whilst the proportion of sand in the sediments did not have a significant impact on egg‐to‐emergence survival (mean survival per treatment 75%–79%), it significantly affected the timing of larval emergence to the water column; early emergence was detected in treatments with elevated sand content (on average, 50% emergence after 12–13 days versus 19 days in the control). Similar to findings from salmonid studies, these results suggest high sand content in spawning gravels can influence timing of larval emergence and potentially cyprinid lithophilic fish survival. [ABSTRACT FROM AUTHOR]

Populations of the European shads Alosa alosa (Linnaeus, 1758) and Alosa fallax Lacépède, 1800 (Alosa spp.) are protected under legislation because of their vulnerability to human disturbances. In particular, river impoundments block their upstream migration, preventing access to spawning areas. Knowledge on the spatial extent of their spawning is important for informing conservation and river management plans.Determining the spatial extent of Alosa spp. spawning is challenging. They enter rivers over a 2‐3‐month period and the species potentially migrate different distances upstream. Capture and handling can be problematic, spawning events generally occur at night, and kick‐sampling for eggs is limited to shallow water. Assessing their spatial extent of spawning could, however, incorporate non‐invasive sampling tools, such as environmental DNA (eDNA).An eDNA assay for Alosa spp. was successfully developed, based on the cytochrome c oxidase subunit I gene segment and quantitative polymerase chain reaction (qPCR). Application in spring 2017 to the River Teme (River Severn catchment, western England) revealed high sensitivity in both laboratory and field trials. Field data indicated Alosa spp. spawning between May and June, with migrants mainly restricted to areas downstream of the final impoundment.eDNA can thus be used as a non‐invasive sampling tool to determine the freshwater distribution of these fishes in Europe, enhancing their conservation at local and regional scales. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying feeding interactions between nonindigenous and indigenous fishes in invaded fish communities is important for determining how introduced species integrate into native food webs. Here, the trophic interactions of invasive 0+ European barbel Barbus barbus (L.) and the three other principal 0+ fishes in the community, Squalius cephalus (L.), Leuciscus leuciscus (L.) and Phoxinus phoxinus (L.), were investigated in the River Teme, a River Severn tributary in Western England. B. barbus has been present in the River Teme for approximately 40 years. Analyses of stomach contents from samples collected from three sites between June and September 2015 revealed that, overall, fishes displayed a generalist feeding strategy, with most prey having low frequency of selection. Relationships of diet composition versus body length and gape height were species‐specific, with increasing dietary specialisms apparent as the 0+ fishes increased in length and gape height. The trophic niche size of invasive B. barbus was always significantly smaller than S. cephalus and L. leuciscus and was significantly smaller than P. phoxinus at two sites. This was primarily due to differences in the functional morphology of the fishes; 0+ B. barbus were generally restricted to foraging on the benthos, whereas the other fishes were able to forage on prey present throughout the water column. Nevertheless, the invasive B. barbus were exploiting very similar prey items to populations in their native range, suggesting these invaders were strongly pre‐adapted to the River Teme and this arguably facilitated their establishment and invasion. [ABSTRACT FROM AUTHOR]

Abstract: Recruitment of salmonids is a result of density‐dependent factors, specifically egg production in the previous year, and density‐independent environmental processes driven by discharge and temperature. With the plethora of knowledge on major drivers of Atlantic salmon Salmo salar and brown trout Salmo trutta recruitment, there is a requirement to explore less known species, such as European grayling Thymallus thymallus, whose postemergence time coincides with period of increasing temperature and low discharge. This study assessed drivers of grayling recruitment in a southern English chalk stream, a system vulnerable to discharge and temperature alterations under future climate change predictions. The analyses explored age 0+ grayling survival in relation to conspecific and heterospecific densities and discharge‐ and temperature‐derived factors. The final mixed‐effects model revealed a positive relationship between age 0+ grayling survival and incubation temperature anomaly and age 0+ trout abundance. Similarly, postincubation temperature anomaly had a positive effect on 0+ grayling survival, but only up to a threshold temperature of 13.5°C, beyond which it had a negative effect. In contrast, increasing number of days with low discharge postincubation negatively influenced age 0+ grayling survival, with no evidence of an effect of elevated discharges following spawning. Our results emphasise the importance of maintaining natural discharge regimes in salmonid rivers by tackling multiple stressors operating at the catchment scale, including land and water use to mitigate for predicted climate driven changes. In addition, further research on recruitment drivers in less stable, rain‐fed systems, is required. [ABSTRACT FROM AUTHOR]

The article focuses on the biological invasions and the cause of biodiversity crisis. Topics include the efforts made to control invasive species aimed at reducing the negative ecological and economic impacts and several methods for Invasive species management for hunting and chemical control measure.

A general framework is presented that should enhance our understanding of how intrinsic factors, such as body size, and extrinsic factors, such as climate, affect the dynamics and demographics of fish populations. Effects of intrinsic factors, notably studies relating juvenile Atlantic salmon Salmo salar body size to their probability to return as an adult, are often context‐dependent and anecdotal, due to data constraints. By merit of its flexible specification, this framework should admit datasets with a range of situation‐specific nuances, collected using different approaches, and thereby deliver more general and robust findings for more effective population management. [ABSTRACT FROM AUTHOR]

Abstract: Invasive species can cause substantial ecological impacts on native biodiversity. While ecological theory attempts to explain the processes involved in the trophic integration of invaders into native food webs and their competitive impacts on resident species, results are equivocal. In addition, quantifying the relative strength of impacts from non‐native species (interspecific competition) versus the release of native conspecifics (intraspecific competition) is important but rarely completed. Two model non‐native fishes, the globally invasive Cyprinus carpio and Carassius auratus, and the model native fish Tinca tinca, were used in a pond experiment to test how increased intra‐ and interspecific competition influenced trophic niches and somatic growth rates. This was complemented by samples collected from three natural fish communities where the model fishes were present. The isotopic niche, calculated using stable isotope data, represented the trophic niche. The pond experiment used additive and substitutive treatments to quantify the trophic niche variation that resulted from intra‐ and interspecific competitive interactions. Although the trophic niche sizes of the model species were not significantly altered by any competitive treatment, they all resulted in patterns of interspecific niche divergence. Increased interspecific competition caused the trophic niche of T. tinca to shift to a significantly higher trophic position, whereas intraspecific competition caused its position to shift towards elevated δ13C. These patterns were independent of impacts on fish growth rates, which were only significantly altered when interspecific competition was elevated. In the natural fish communities, patterns of trophic niche partitioning between the model fishes was evident, with no niche sharing. Comparison of these results with those of the experiment revealed the most similar results between the two approaches were for the niche partitioning between sympatric T. tinca and C. carpio. These results indicate that trophic niche divergence facilitates the integration of introduced species into food webs, but there are differences in how this manifests between introductions that increase inter‐ and intraspecific competition. In entirety, these results suggest that the initial ecological response to an introduction appears to be a trophic re‐organisation of the food web that minimises the trophic interactions between competing species. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Variation in movement between individuals can have important ecological effects on populations and ecosystems, yet the factors driving differences in movement and their consequences remain poorly understood. Here, individual variability in the movements of juvenile (age 0 + and 1 + year) pike Esox lucius was assessed using passive integrated transponder ( PIT) telemetry in off-channel nursery areas over a 26- month period. Differences in the movement patterns of individuals were tested against their body sizes, ages, growth rates and trophic positions using data collected through a combination of catch-and-release sampling and stable isotope analyses. Results revealed that variation in movement between individuals was affected by age, with 1 + individuals moving more than individuals of age 0 + , but not length. Individuals whose TP was low on their initial capture event moved significantly less than those with a higher initial TP. Individuals that moved more grew faster and achieved a higher final TP. These results suggest that higher activity (i.e., increased movement) increases resource acquisition that enhances growth rates, which could ultimately maximise individual performances. [ABSTRACT FROM AUTHOR]

Comparative functional responses ( FRs) can predict impacts of invasive species, including piscivorous fishes, via quantifying their depletion of native food resources as a function of prey density. The utility of FRs for predicting impacts on prey populations by invasive fishes of different trophic guilds was tested here by comparing the FRs of the invaders Cyprinus carpio and Carassius auratus, with three native, trophically analogous fishes, Barbus barbus, Squalius cephalus and Tinca tinca. Chironomid larvae and Gammarus pulex were used as prey items. Predictions, developed from studies on the foraging of C. carpio and C. auratus in the literature, were that the invaders would have significantly higher consumption rates for chironomids than the native fishes, but not for G. pulex. Mean consumption rates for chironomids were significantly lower for both invaders than B. barbus and S. cephalus, but were similar to T. tinca. Barbus barbus had a significantly lower consumption rate of G. pulex than both invaders, but there were no significant differences between S. cephalus, T. tinca and the invaders. All FRs were type II, with FR curves for the invaders preying upon chironomids never being significantly higher than the native fishes, contrary to predictions. For G. pulex, some significant differences were apparent between the invaders and native fishes, but again were contrary to predictions. These results indicated that when predation impacts of invasive fishes could also be a function of their population density and body sizes, these parameters should be incorporated into FR models to improve impact predictions. [ABSTRACT FROM AUTHOR]

Despite the growing evidence for individual variation in trophic niche within populations, its potential indirect effects on ecosystem processes remains poorly understood. In particular, few studies have investigated how intraspecific trophic variability can modulate the effects of consumers on ecosystems through potential changes in nutrient excretion rates. Here, we first quantified the level of intraspecific trophic variability in 11 wild populations of the omnivorous fish Lepomis gibbosus. Outputs from stomach content and stable isotope analyses revealed that the degree of trophic specialization and trophic positions were highly variable between and within these wild populations. There was intrapopulation variation in trophic position of more than one trophic level, suggesting that individuals consumed a range of plant and animal resources. We then experimentally manipulated intraspecific trophic variability to assess how it can modulate consumer-mediated nutrient effects on relevant processes of ecosystem functioning. Specifically, three food sources varying in nutrient quality (e.g. plant material, macro-invertebrate and fish meat) were used individually or in combination to simulate seven diet treatments. Results indicated that intraspecific variability in growth and nitrogen excretion rates were more related to the composition of the diet rather than the degree of specialization, and increased with the trophic position of the diet consumed. We subsequently used microcosms and showed that critical ecosystem functions, such as primary production and community respiration, were affected by the variability in excretory products, and this effect was biomass-dependent. These results highlight the importance of considering variation within species to better assess the effects of individuals on ecosystems and, more specifically, the effects of consumer-mediated nutrient recycling because the body size and the trophic ecology of individuals are affected by a large spectrum of natural and human-induced environmental changes. [ABSTRACT FROM AUTHOR]

Predicting the ecological consequences of invasions by non-native species is a fundamental aspect of their risk-based management. As impacts can include the negative consequences of resource sharing with native species, the application of in situ cohabitation field experiments can test hypotheses relating to invasion ecology via competitive interactions and processes. As fishes are adaptable and tractable experimental organisms, they are strong model species for use in studies on competitive interactions., The trophic consequences of invasion by two globally invasive freshwater fishes, common carp Cyprinus carpio and goldfish Carassius auratus, were tested on the threatened native fish crucian carp Carassius carassius. Cohabitation experiments, completed in pond enclosures, used all species in allopatric and sympatric treatments using a substitutive design where the number of fish per treatment was kept constant. Stable isotope analysis (δ13C and δ15N) assessed alterations in the trophic ecology of each species across treatments, with growth rates used to assess any consequent impacts on the fish., When in sympatry with C. auratus and C. carassius, the Cy. carpio isotopic niche was at a significantly lower trophic position compared to allopatry. This resulted in niche overlap with C. auratus, while for C. carassius, their isotopic niche shifted to a higher trophic position compared with allopatry. The growth rate of Cy. carpio was always significantly higher in sympatry than in allopatry, whereas growth rates for C. carassius and C. auratus were significantly depressed in Cy. carpio presence. In contrast, the isotopic niche sizes and positions and growth rates of the Carassius fishes were not significantly different between allopatry and when they cohabited., Plasticity in the isotopic niche of Cy. carpio resulted in significant alterations in their trophic position between allopatry and sympatry and, when coupled with their depressed growth in allopatry, suggests the competitive processes driving this were intra-specific rather than inter-specific. This then resulted in detrimental impacts on cohabiting Carassius fishes. These results emphasise that ecological consequences of Cy. carpio in invaded freshwaters include impacts on the trophic ecology of native fishes. [ABSTRACT FROM AUTHOR]

The trophic ecology of invasive species has important implications for their impacts on recipient ecosystems, with omnivorous invaders potentially affecting native species and processes over multiple trophic levels. The trophic ecology of invaders might be affected by both their body size and the characteristics of their habitat due to variation in energy requirements and resource availability., Here, using stable-isotope analysis, we investigated the trophic ecology of the invasive crayfish Procambarus clarkii in 15 populations in southwest France over a gradient of individual (crayfish body size), population (crayfish abundance) and ecosystem (lake size, productivity and predation pressure) characteristics. We predicted that population niche width, level of omnivory and trophic position of individuals would change with abiotic and biotic conditions, but that these relationships would vary with lake size., The trophic position of individual crayfish increased with body size in lakes with low productivity, but decreased with body size in more productive lakes. As crayfish abundance increased (and therefore potential intraspecific competition), individual trophic position and population niche width decreased. This was most apparent in smaller lakes, suggesting it related to an increase in encounter rates with conspecifics., Body size, population abundance, lake size and lake productivity influenced the trophic ecology of invasive crayfish, which can affect their interactions with native species. Our results demonstrated that the trophic ecology of invasive species can be variable across invaded landscapes, with implications for their ecological impacts on native communities. This emphasizes the importance of characterising the diet of invasive species across their non-native range and environmental gradients to better predict and manage their impacts. [ABSTRACT FROM AUTHOR]

Predictions of the identities and ecological impacts of invasive alien species are critical for risk assessment, but presently we lack universal and standardized metrics that reliably predict the likelihood and degree of impact of such invaders (i.e. measurable changes in populations of affected species). This need is especially pressing for emerging and potential future invaders that have no invasion history. Such a metric would also ideally apply across diverse taxonomic and trophic groups., We derive a new metric of invader ecological impact that blends: (i) the classic Functional Response ( FR; consumer per capita effect) and Numerical Response ( NR; consumer population response) approaches to determining consumer impact, that is, the Total Response ( TR = FR × NR), with; (ii) the 'Parker-Lonsdale equation' for invader impact, where Impact = Range × Abundance × Effect ( per capita effect), into; (iii) a new metric, Relative Impact Potential ( RIP), where RIP = FR × Abundance. The RIP metric is an invader/native ratio, where values >1 predict that invader ecological impact will occur, and increasing values above 1 indicate increasing impact. In addition, the invader/invader RIP ratio allows comparisons of the ecological impacts of different invaders., Across a diverse range of trophic and taxonomic groups, including predators, herbivores, animals and plants (22 invader/native systems with 47 individual comparisons), high-impact invaders were significantly associated with higher FRs compared to native trophic analogues. However, the RIP metric substantially improves this association, with 100% predictive power of high-impact invaders., Further, RIP scores were significantly and positively correlated with two independent ecological impact scores for invaders, allowing prediction of the degree of impact of invasive alien species with the RIP metric. Finally, invader/invader RIP scores were also successful in identifying and associating with higher impacting invasive alien species., Synthesis and applications. The Relative Impact Potential metric combines the per capita effects of invaders with their abundances, relative to trophically analogous natives, and is successful in predicting the likelihood and degree of ecological impact caused by invasive alien species. As the metric constitutes readily measurable features of individuals, populations and species across abiotic and biotic context-dependencies, even emerging and potential future invasive alien species can be assessed. The Relative Impact Potential metric can be rapidly utilized by scientists and practitioners and could inform policy and management of invasive alien species across diverse taxonomic and trophic groups. [ABSTRACT FROM AUTHOR]

The crossing of freshwater ecosystem boundaries by marine-derived nutrients ( MDN) is usually associated with migratory salmonid fishes returning to natal rivers. An alternative source of MDN in fresh waters is the widespread use of pelletised marine fishmeal ('pellets') by freshwater anglers as they target large-bodied cyprinid fishes, such as European barbel Barbus barbus., Here, the trophic consequences of MDN from pellets for riverine cyprinid fishes were tested. Approaches used stable isotope analyses in controlled and wild scenarios, using B. barbus and chub Squalius cephalus as model species. The isotopic niche, measured as standard ellipse area, was used to assess trophic niche size, and mixing models predicted the extent to which MDN contributed to fish diet., In experimental mesocosms, B. barbus fed low volumes of pellets ( c. 3 per fish) for 130 days had isotopic niche sizes that were up to four times larger than a control and 'medium' (6 per fish) and 'high' pellet (12 per fish) treatments. Somatic growth rates were significantly higher in the 'medium' and 'high' treatments. In pond enclosure experiments, when juvenile B. barbus and S. cephalus were fed pellets daily for 100 days, there was a substantial and significant shift in the position of their isotopic niche compared to controls with no pellets fed. However, for each species, there were no significant differences in their somatic growth rates in the presence/absence of pellets., In a lowland river, high proportions of MDN contributed to the diet of B. barbus and S. cephalus captured by angling, but with substantial individual variability in those captured by electric fishing. Across all B. barbus >400 mm, MDN dietary contributions ranged between 9% and 71%. This suggested some individual diet specialisations within their population that was associated with feeding on this angler subsidy and that also resulted in a significant increase in the size of their population isotopic niche., These results suggested that when pellets containing MDN are used in freshwater angling, they are consumed and assimilated by cyprinid fishes, influencing individual and population trophic positions, and isotopic niche sizes and dietary specialisations. The results also suggested that the extent to which individuals specialise in feeding on pellets potentially influences their vulnerability to capture by anglers. [ABSTRACT FROM AUTHOR]

Background While protein is the predominant solute measured in plasma or serum by a refractometer, nonprotein substances also contribute to the angle of refraction. There is debate in the current literature regarding which nonprotein substances cause factitiously high refractometric total protein measurements, as compared to the biuret assay. Objectives The purpose of the study was to determine if the blood of azotemic animals, specifically with increased blood urea concentration, will have significantly higher refractometric total protein concentrations compared to the total protein concentrations measured by biuret assay. Methods A prospective case series was conducted by collecting data from azotemic ( n = 26) and nonazotemic ( n = 34) dogs and cats. In addition, an in vitro study was performed where urea was added to an enhanced electrolyte solution at increasing concentrations, and total protein was assessed by both the refractometer and spectrophotometer. Statistical analysis was performed to determine the effect of urea. Results The refractometric total protein measurement showed a positive bias when compared to the biuret protein measurement in both groups, but the bias was higher in the azotemic group vs the nonazotemic group. The mean difference in total protein measurements of the nonazotemic group (0.59 g/dL) was significantly less ( P < .01) than the mean difference of the azotemic group (0.95 g/dL). The in vitro experiment revealed a positive bias with a proportional error. Conclusions This study demonstrated that increasing concentrations of urea significantly increased the total protein concentration measured by the refractometer as compared to the biuret assay, both in vivo and in vitro. [ABSTRACT FROM AUTHOR]

Stable isotope analysis (SIA) was used to examine the isotopic relationships between dorsal muscle and fin, scale and epidermal mucus in pike Esox lucius. δ13C and δ15N varied predictably within each tissue pairing, with conversion factors calculated for the surrogate tissues, enabling their application to the non‐lethal sampling of E. lucius for SIA. [ABSTRACT FROM AUTHOR]