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Spawning migrations of fishes are common case studies for examining the magnitude of resource subsidies; however, no studies have evaluated this phenomenon in iteroparous migrations of the Catostomid (i.e. suckers) genus Moxostoma (i.e. redhorses). Fish resource subsidies are usually represented as nitrogen (N) and phosphorus (P), and migratory redhorses can deliver these nutrients via excretion and eggs. We evaluated whether redhorses deliver N and P subsidies to Brasstown Creek—a fourth‐order stream in the Southeastern US—during their spawning migrations. We measured individual‐level nutrient contributions from excretion and eggs and extrapolated those values to estimate nutrient contributions based on daily abundances of migratory redhorses. We compared daily nutrient inputs to daily nutrient exports to determine the potential for resource subsidies. We estimated 71.6 kg of N and 4.4 kg of P were delivered to Brasstown Creek over a 98‐day migration period. N input exceeded N export on 15 of the 98 days, but phosphorus input never exceeded P export. Eggs accounted for the majority of N and P input at 51% and 100% respectively. Redhorse N inputs peaked in the spring during a period of low N exports, suggesting that redhorse subsidies were delivered during times of elevated nutrient demand and relatively low nutrient availability. Suckers receive little management attention, and several species of redhorses are imperilled. Given our conclusion that redhorses provide a temporally critical N subsidy that is likely available to multiple consumers in freshwater food webs, this study provides compelling motivation for increased conservation of these species and their migrations. [ABSTRACT FROM AUTHOR]

Spawning phenology and associated migrations of fishes are often regulated by factors such as temperature and stream discharge, but flow regulation of mainstem rivers coupled with climate change might disrupt these cues and affect fitness. Flannelmouth sucker (Catostomus latipinnis) persisting in heavily modified river networks are known to spawn in tributaries that might provide better spawning habitat than neighboring mainstem rivers subject to habitat degradation (e.g., embedded sediments, altered thermal regimes, and disconnected floodplains). PIT tag data and radio telemetry were used to quantify the timing and duration of flannelmouth sucker tributary spawning migrations in relation to environmental cues in McElmo Creek, a tributary of the San Juan River in the American Southwest. We also tested the extent of the tributary migration and assessed mainstem movements prior to and after tributary migrations. Additionally, multiyear data sets of PIT detections from other tributaries in the Colorado River basin were used to quantify interannual and cross‐site variation in the timing of flannelmouth sucker spawning migrations in relation to environmental cues. The arrival and residence times of fish spawning in McElmo Creek varied among years, with earlier migration and a 3‐week increase in residence time in relatively wet years compared to drier years. Classification tree analysis suggested a combination of discharge‐ and temperature‐determined arrival timing. Of fish PIT tagged in the fall, 56% tagged within 10 km of McElmo Creek spawned in the tributary the following spring, as did 60% of radio‐tagged fish, with a decline in its use corresponding to increased distance of tagging location. A broader analysis of four tributaries in the Colorado River basin, including McElmo Creek, found photoperiod and temperature of tributary and mainstem rivers were the most important variables in determining migration timing, but tributary and mainstem discharge also aided in classification success. The largest tributary, the Little Colorado River, had more residential fish or fish that stayed for longer periods (median = 30 days), whereas McElmo Creek fish stayed an average of just 10 days in 2022. Our results generally suggest that higher discharge, across years or across sites, results in extended use of tributaries by flannelmouth suckers. Conservation actions that limit water extraction and maintain natural flow regimes in tributaries, while maintaining open connection with mainstem rivers, may benefit migratory species, including flannelmouth suckers. [ABSTRACT FROM AUTHOR]

During spawning migrations, adult Pacific salmon (Oncorhynchus spp.) must undergo a suite of morphological and physiological changes to prepare for spawning. Modern genomics approaches can be used to conduct nonlethal assessments of physiology and infectious agent presence in migrating salmon. We investigated gene expression changes and shifts in the prevalence and load of 18 infectious agents in the gill tissue of adult sockeye salmon (Oncorhynchus nerka) during the final 50 km of their freshwater migration. We found 77% of fish successfully migrated to spawning grounds. Fish exhibited a decrease in thermal stress gene expression following lake migration, consistent with the hypothesized benefits of thermal refugia during spawning migrations. We also found evidence of cooler water selection (∼1.5 °C) during lake migration for fish with higher infection burden, suggesting behavioural changes for fish suffering higher infection burdens and potential benefits of thermal refuges for reducing the negative impacts of infection for migrating salmon. Collectively, our findings provide further evidence that fish behaviourally regulate exposure to stressful conditions such as high temperature that may undermine survival. [ABSTRACT FROM AUTHOR]

Earlier spring warming and anadromous fish migrations prompted by climate change are linked to shorter freshwater residency. Impacts of phenological change on anadromous fish populations are poorly understood with limited studies focused on iteroparous non-salmonids. We assessed freshwater residence time and reproductive success in an iteroparous clupeid, alewife (Alosa pseudoharengus) using a pedigree analysis and otolith-based spawning dates from captured juveniles. The primary objectives were to (1) estimate adult spawning duration in a freshwater pond (freshwater residence time) and (2) evaluate adult freshwater residence time, arrival date, length, sex, and reproductive success across 2 years in one system. Estimated freshwater residence times varied widely (1–64 days), and longer residence times were associated with earlier arrival dates, higher reproductive success, and more mating events. Longer freshwater residence times may allow alewife to spawn with more mates, produce more gametes, and experience a range of spawning and nursery conditions. Plasticity in alewife freshwater residence time could support earlier and shorter migration periods but may result in lower reproductive output if adults spend less time in freshwater ponds. [ABSTRACT FROM AUTHOR]

The ability for migratory fishes to move commonly limiting resources such as nitrogen (N) and phosphorus (P) between discrete environments can have pronounced effects on recipient ecosystems. To further understand the geographic and taxonomic scope of migratory fish resource subsidies, we quantified N and P subsidies delivered by adfluvial suckers (Smallmouth Buffalo, Ictiobus bubalus) via excretion, eggs and carcasses to a small oligotrophic stream during their annual spawning migration. We also compared nutrient inputs from migrant buffalo with watershed nutrient export to assess the likelihood that delivered nutrients were ecologically important. We estimated that approximately 67,000 buffalo delivered 730 kg of N and 80 kg of P to Citico Creek as a result of excretion and egg subsidies across three migration waves. We estimated that carcasses delivered negligible amounts of N and P due to extremely low retention. The ratio of migrant inputs (Mw) to system export (Ew; Mw/Ew) varied amongst three migration waves and compounds (i.e. dissolved inorganic nitrogen, ammonium and soluble reactive phosphorus), with values for Mw/Ew ranging from 0.25 to 5.10, reflecting the potential of nutrient subsidies to exceed nutrients exported from the system under certain conditions. Our findings suggest that suckers have the potential to deliver large resource subsidies to their spawning habitats and that these subsidies may be ecologically important, thus warranting additional consideration of the functional relevance of nongame fishes and their migrations. [ABSTRACT FROM AUTHOR]

Lake Tana is one of the largest freshwater bodies in East Africa, with its 18 Labeobarbus species being the world's last remaining Cyprinid species flock. Many of them feed and grow in Lake Tana and migrate to the tributaries of the lake to reproduce. The construction of irrigation structures is increasing in the tributary rivers, however, without due attention being given to the spawning migration routes and critical spawning habitats. The present study examined the impacts of the Gilgel Abay Weir on the spawning migration of these species and discusses possible interactions with other human‐related impacts and potential mitigation measures. Fish samples were taken twice monthly from July to November 2019, both below and above the weir with gillnets. No spawning migration to the upstream of the weir has occurred since its construction, with the weir being a barrier to their spawning migration route of Lake Tana fisheries. Of the 11 species caught, only Labeobarbus intermedius (a habitat generalist) was observed above the weir, with the species diversity being significantly higher below the weir (p < 0.05) than above it. The total number of species observed during the present study was also lower, compared with previous reports. The fisheries impacts may also have been exacerbated by other contributing factors such as habitat destruction, river system degradation, destructive fishing practices and weed infestation, a situation which may lead to the collapse of Lake Tana fisheries and ultimately the lake's ecology. Reversing this situation is crucial, noting the implementation of such measures as habitat restoration and improvement, replacing the weir with environmentally friendly technologies, prohibition of illegal fishing, and construction of fish ladders could definitely alleviate the observed negative impacts of the Gilgel Abay Weir on the fisheries. [ABSTRACT FROM AUTHOR]

Simple Summary: Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail. Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations. [ABSTRACT FROM AUTHOR]

The involvement of the preoptico- or hypothalamo-hypophysial neurosecretory system (HHNS) in the initiation of migration processes in fish has been discovered, based on the results of ecologo-histophysiological studies using light microscopy, immunohistochemistry, electron microscopy and morphometry, thus confirming the hypothesis proposed by N.L. Gerbilsky and A.L. Polenov about the nature of the migratory impulse in fish. It has been demonstrated for the first time that in genetically distant but ecologically similar anadromous migratory fish species with different breeding seasons (spring-spawning Russian sturgeon, stellate sturgeon, and fall-spawning pink salmon), the synthesis of neurosecretory products is activated in neurosecretory cells at the beginning of the migration process, followed by their transport and accumulation in the neurohypophysis. At the same time, neurosecretory products have been shown to be secreted into the cerebrospinal liquid of the 3rd ventricle. Thus, the HHNS has a complex synchronous effect, which includes: (1) the neurotropic action of nonapeptide neurohormones on behavior centers of the central nervous system (CNS), causing a dominant state of CNS excitation originally called the "migratory impulse", (2) osmoregulatory imbalance during the marine "fattening" period, and (3) the cessation of the known antigonadotropic action of nonapeptide neurohormones, thus promoting the switching of the organism to an energy-intensive type of energy metabolism. A comparative analysis of our own and literature data confirms the communality of this metabolic mechanism in fish, thus allowing it to be considered as the most important phylogenetic adaptation (aromorphosis) aimed at achieving biological progress of species. A leading role in the subsequent navigational mechanisms of homing, widely covered in the world literature, is played by the luliberinergic centers of the hypothalamus. The ecological diversity of these mechanisms at the population level and the presence of straying in fish allow considering them as microevolutionary specializations. Further analysis of HHNS functions during fish migrations and spawning will help establish the basic principle of integration of these processes at the given ontogenetic stage, which is of prime importance for the existence of species. [ABSTRACT FROM AUTHOR]

Non-physical deterrence and guidance technologies are sought after for management of invasive fish, but they have rarely been tested in the field during natural fish migrations. We tested the efficacy of a semi-portable deterrence and guidance system (DGS) that used vertical electrodes producing low-voltage electric field to deter the upstream spawning migration of common carp (Cyprinus carpio Linnaeus, 1758) and direct them into a trap (5 m by 25 m enclosure with a net) in a natural stream. The behavior of carp was monitored using Passive Integrated Transponder (PIT) tags and antennas. The migration began on 7 April 2019, when water temperature was 5 °C and lasted through May 2019. Performance of the DGS was evaluated during three separate periods, between which the DGS was briefly deactivated. During the first period (4/7-4/19), 559 unique carp challenged the DGS. Of those, 10 were able to cross the DGS (98.2% efficacy), 234 (41.8%) were directed into the trap and 54 were captured. The mean number of days each carp was detected at the DGS was 2.6. During the second period (4/25-5/7), 562 unique PIT carp challenged the DGS. Of those, 24 were able to cross it (95.8% efficacy), 241 (42.9%) were directed into the trap and 148 were captured. Mean time at the DGS was 5.6 days. During the third period (5/7-5/15), 147 PIT carp challenged the DGS and 2 were able to cross it (98.6% efficiency). The mean time at the DGS was 5.5 days. Due to decreasing numbers of carp during the third period, we did not attempt to direct them into the trap for removal. Stream discharge during our test exceeded the 11-year average by 2-3 times; the floodplain was periodically flooded, and it is possible that some carp migrated around the DGS rather than through it. Overall, our results suggest that DGS that generate mild, electric field can be useful in deterring seasonal migrations of common carp and facilitating new removal strategies. The semi-portable nature of the system and low site-engineering requirements allow for rapid deployments and targeted applications during seasonal migrations. [ABSTRACT FROM AUTHOR]

Life history events, from mating and voltinism to migration and emergence, are governed by external and historically predictable environmental factors. The ways humans have altered natural environments during the Anthropocene have created myriad and compounding changes to these historically predictable environmental cues. Over the past few decades, there has been an increased interest in the control temperature exerts on life history events as concern over climate change has increased. However, temperature is not the only life history cue that humans have altered. In stream ecosystems, flow and light serve as important life history cues in addition to temperature. The timing and magnitude of peak flows can trigger migrations, decreases in stream temperature may cause a stream insect to enter diapause, and photoperiod appears to prompt spawning in some species of fish. Two or more of these cues may interact with one another in complex and sometimes unpredictable ways. Large dams and increasing impervious cover in urban ecosystems have modified flows and altered the timing of spawning and migration in fish. Precipitation draining hot impervious surfaces increases stream temperature and adds variability to the general pattern of stream warming from climate change. The addition of artificial light in urban and suburban areas is bright enough to eliminate or dampen the photoperiod signal and has resulted in caddisfly emergence becoming acyclical. The resulting changes in the timing of life history events also have the potential to influence the evolutionary trajectory of an organism and its interactions with other species. This paper offers a review and conceptual framework for future research into how flow, temperature, and light interact to drive life history events of stream organisms and how humans have changed these cues. We then present some of the potential evolutionary and ecological consequences of altered life history events, and conclude by highlighting what we perceive to be the most pressing research needs. [ABSTRACT FROM AUTHOR]

Choosing whether or not to migrate is an important life history decision for many fishes. Here we combine data from physical captures and detections on autonomous passive integrated transponder (PIT) tag antennas to study migration in an endangered fish, the humpback chub (Gila cypha). We develop hidden Markov mark–recapture models with and without antenna detections and find that the model fit without antenna detections misses a large proportion of fish and underestimates migration and survival probabilities. We then assess survival and growth differences associated with life history strategy and migration for different demographic groups (small male, small female, large male, large female). We find large differences in survival according to life history strategy, where residents had much lower overwinter survival than migrants. However, within the migratory life history strategy, survival and growth were similar for active migrants and skipped migrants for all demographic groups. We discuss some common challenges to incorporating detections from autonomous antennas into population models and demonstrate how these data can provide insight about fish movement and life history strategies. [ABSTRACT FROM AUTHOR]

Anthropic exploitation of soil, forests, and water bodies contributes to the modification of river flow regimes and sediment transport dynamics, thus affecting fish habitats and water quality. In the last decades, the increasing awareness of the consequences of such modifications on natural contexts has stimulated researchers and practitioners to provide more innovative solutions aiming at enhancing river biodiversity. Recently developed eco-friendly approaches for river restoration evidenced the necessity of more careful planning and management of natural resources, including the maintenance and enhancement of fish species' habitats. The present study aims at assessing the interaction of different types of wood- and rock-made instream structures with river flow regimes and habitat requirements of many freshwater fish species, accounting for the influence of downstream scour potholes to facilitate fish species migration and spawning. Experimental tests showed that instream structures cause deep scour holes characterized by low flow velocity zones, thus decreasing the required fish swimming power and potentially providing suitable conditions for the migration of freshwater fish species. Using dimensional analysis, empirical relationships were derived to optimize the longitudinal distance between subsequent instream structures, accounting for the effects of various structure geometries and fish biological characteristics. Nevertheless, further investigation is needed to validate the proposed relationships at prototype scale. This study ultimately contributes at providing tools to design river instream structures favorable to fish migration. • The effects of different types of instream structures on fish swimming hydrodynamics have been investigated. • The analysis was conducted at model scale under identical fish Froude numbers F f. • Results showed that instream structures facilitate the migration of freshwater fish species. • Fish white muscles weight increases with the fish weight resulting in an increase of the available fish power. [ABSTRACT FROM AUTHOR]

Many commercially important groupers (Epinephelidae) form fish spawning aggregations (FSA) at specific sites where the spawning stock is concentrated to spawn within a couple of months each year. The concentrated nature and short duration of these FSAs render these species susceptible to fishing pressure leading to their disappearance or decrease in the number of spawning fish. As a result of directed fishing during FSAs many large-bodied groupers are now International Union for Conservation of Nature (IUCN) Red-listed. Puerto Rico within the US Caribbean is where protective measures have been designated for the recovery of grouper species that aggregate to spawn, in particular the threatened (Endangered Species Act) Nassau grouper (Epinephelus striatus) and sustainability of the red hind (Epinephelus guttatus). The regulations at three marine managed areas (MMAs) off the west coast of Puerto Rico, have been focused on reproductive seasons, through seasonal closures. However, these management zone boundaries may fail to fully encompass fish migration paths and temporal variability in FSAs, exposing threatened species like the Nassau grouper to incidental fishing and poaching. In addition to traditional methods used to study FSAs, passive acoustics represent a relatively new approach to assess the presence and spatio-temporal distribution of aggregating species. Sound production is common among many fishes, including the Nassau and red hind groupers, and is most often associated with courtship and spawning behavior. In this study, we present the results from the deployment of an autonomous surface vehicle equipped with a passive acoustic monitoring system along the insular shelf break that surveyed the MMAs during their peak spawning season. Our observations suggest that known FSA sites are critical habitat for both species and the existence of potential, previously unknown FSAs for multiple grouper species as well, highlighting the importance of spatial and temporal expansion of existing regulations. [ABSTRACT FROM AUTHOR]

Body condition and reproduction data are broadly used to assess the health status of fish because of its implications for recruitment and ecosystem structure. Sardina pilchardus is a small pelagic distributed throughout both Mediterranean and Eastern Atlantic. Seasonal trend analysis of energy storage and reproduction was carried out in sardines from two areas along the Atlantic-Mediterranean transition: Southern Portugal-Gulf of Cádiz (POR-GC) (Atlantic Ocean) and Alboran Sea (Alb) (Mediterranean Sea) from 2019 to 2021. Energetic condition was estimated using tissue and mesenteric fat content, hepatosomatic index (HSI), and the relative condition factor (Kn). Sex, reproductive developmental stage, and gonadosomatic index (GSI) were also obtained. In addition, the oceanographic and meteorological characteristics of the areas were analysed. Results showed that seasonal Kn, tissue and mesenteric fat content, and HSI values of POR-GC specimens exceeded Alb's with summer arrival, period in which sardine acquires reserves to allocate them to reproduction. These differences could be associated to greater productivity of the former area mainly due to rivers discharges and trade winds intensification during summer (from July to September). Furthermore, gonad maturation of POR-GC stock occurred before the Alb. However, no spawning capable individuals were identified until February in POR-GC. In contrast, in Alb it was observed a remarkable fraction of spawning capable and active spawner individuals in October. We hypothesized the migration of mature individuals from POR-GC to the spawning areas located in the Alb. Seasonal genetic population studies are required to untangle it and reliably evaluate the environmental effect on the stocks. • Atlantic sardines show better somatic condition than Alboran individuals at storage period. • Productivity in the Atlantic area is greater than in the Alboran Sea. • Rivers discharges and summer's trade winds intensification could be its main cause. • Results suggest the migration of mature fish from the Atlantic towards the Alboran. • Seasonal genetic population studies are required. [ABSTRACT FROM AUTHOR]

• Timing and duration of herring coastal spawning migration differ by fish age and size. • Older fish spawn earlier and remain in frozen surface Akkeshi Lake waters. • Younger fish swim shallower than older fish through the day and night. • Different fish ages and length may lead to variable risk and cost in migration. Timing and duration of spawning migration of Pacific herring is known to vary by fish age and length. Older fish mature earlier, but there are few verified examples of differences in the pre-spawning migration of individuals varying in age or size. Here we use sonic tagging to examine the fluctuations in age, fish length and maturity throughout the migration of herring in Akkeshi, Japan. We tracked the horizontal and vertical movement of individual fish using acoustic telemetry and biologging methodology. In Akkeshi Lake, older, larger individuals migrated early, reached peak maturity and remained in the lake until early April, after spawning. The timing and duration of the stay in Akkeshi Lake coincided with the freezing and melting of the surface layer of the lake. Fish tracking indicated no differences in herring in different length classes migrated to and from Akkeshi Lake and offshore and stayed in Akkeshi Bay. For the vertical movement, larger, older herring occupied a wide depth range during diurnal vertical movements as they moved to Akkeshi Lake and to offshore. In contrast, the smaller group occurred frequently in depths shallower than 5 m throughout the day and night. Greater depth variation could result in beneficial effects in predator avoidance and increased prey encounter opportunities though vertical migration. In contrast, smaller herring that were more confined to surface waters could experience more frequent sensing of the environmental cues of spawning grounds such as low-salinity water and olfactory stimulation near the surface. Therefore, there are differences in the benefits and risks during migration for each fish length, which may result in superior spawning grounds and individual mortality in this area. [ABSTRACT FROM AUTHOR]