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The assessment of ecological quality in freshwater ecosystems is a key issue in many countries, but conditions for the development of assessment methodologies are often country-specific. This study proposes a simple methodology for the assessment of the ecological potential of reservoirs based on fish communities using a dataset covering major environmental and pressure gradients in reservoirs in the Czech Republic. Fish data obtained by gillnet sampling were correlated with a proxy of eutrophication as a key indicator of anthropogenic pressure for selecting appropriate fish-based indicators, establishing scoring criteria and developing the index of ecological quality. Expert judgement was also used to select potential fish indicators. Nine indicators were selected for the final fish-based index, fulfilling the criteria required by the Water Framework Directive. Two steps were used to validate the fish-based index quantification of its inter annual stability and sensitivity analysis of individual indicators. Finally, the index was compared to a previously developed general index for Central and Western Europe. Our study demonstrates that a combination of expert judgement and strict validation methods can result in an informative assessment of the ecological conditions, which can help identify conservation and restoration priorities.

European catfish is a large-bodied apex predator, a key species in native areas, but invasive in others where it negatively impacts local aquatic fauna necessitates catfish regulation. However, traditional ichthyological methods face challenges in capturing it. The study presents a detailed description of the efficient long-line method, refined through 48 sampling campaigns across twelve European water bodies. This method proves cost-effective and technically undemanding, requiring an average of 5.6 bait fish to catch one European catfish per day. The long-lines outperform other techniques, with the highest Biomass per unit effort (BPUE) of 6.205 kg of catfish per man-hour and minimal by-catch (0.276 kg per man-hour). In contrast, fyke nets, the second most efficient method, achieve a BPUE of 0.621 kg of catfish per man-hour with 3.953 kg of by-catch per man-hour. To optimize long-line catches, a 15 m distance between branch lines and regular relocation is recommended. Live fish is the most effective bait with no significant differences observed among species. However, earthworms, a less controversial alternative, are also efficient, especially for smaller catfish. Our recapture approach using various ichthyological methods revealed no hook avoidance behavior by catfish after a previous catch or avoidance by a certain part of the population. The long-line method is suitable for population regulation, scientific research, and conservation efforts and is the most effective means of capturing live European catfish., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors.)

Day and nighttime autumn fish abundance and biomass were studied in the pelagic and littoral zones of Lake Sauka in Latvia. Both pelagic methods (hydroacoustics and trawling) revealed significantly higher fish abundance and biomass during the day than at night, especially in deeper zones (below 3 m). Roach (Rutilus rutilus) and Eurasian perch (Perca fluviatilis) dominated the trawl catches during the day, while roach and ruffe dominated at night. Fish smaller than 14 cm strongly dominated in both the trawl catches and hydroacoustic observations. Our hydroacoustic sampling found inhomogeneous pelagic distributions of fish aggregated in big shoals during the day. In the littoral zone, which was sampled by beach seining, both the abundance and biomass were significantly higher at night than during the day. Roach, bleak, and European perch usually dominated in beach seine catches during the day and at night. The daytime pelagic biomass found by hydroacoustics was 62 kg/ha and it decreased to approximately 11 kg/ha at night. The littoral biomass found by beach seining was diurnally opposite, 4 kg/ha during the day and 37 kg/ha at night. It is obvious that diurnal horizontal migrations between pelagic and littoral zones, and shoaling behavior during the daytime are common patterns in the shallow Lake Sauka during the autumn. The study of the spatial distribution of fish is extremely important for the establishment of an appropriate monitoring plan for the purposes of the Water Framework Directive with regard to the morphometry of the lake, the geographical location, and the sampling period of the year. This study also shows that the combination of completely non-invasive hydroacoustic and other methods that are invasive (trawls, beach seines) but not as destructive as gillnets, which are normally used for scientific fish monitoring in Europe, could be a future way forward for fish monitoring., (© 2024 The Authors. Journal of Fish Biology published by John Wiley & Sons Ltd on behalf of Fisheries Society of the British Isles.)

Stable isotope analysis (SIA) is widely used to study trophic ecology and food webs in aquatic ecosystems. In the case of fish, muscle tissue is generally preferred for SIA, and the method is lethal in most cases. We tested whether blood and fin clips can be used as non-lethal alternatives to muscle tissue for examining the isotopic composition of two freshwater predatory fish, European catfish (Silurus glanis) and Northern pike (Esox lucius), species of high value for many freshwater systems as well as invasive species in many others. Blood samples from the caudal vein, anal fin clips, and dorsal muscle obtained by biopsy punch were collected from four catfish and pike populations (14-18 individuals per population). Subsequently, these samples were analyzed for δ13C and δ15N. The effects of alternative tissues, study site, and fish body mass on the isotopic offset were investigated. Both species showed a correlation between the isotopic offset and the tissue type, as well as the study site, but no significant relationship with the body mass. The isotopic offsets between tissues were used to calculate the conversion equations. The results demonstrated that both blood and fin clips are suitable and less invasive alternative to muscle in SIA studies focused on European catfish and Northern pike. Blood provided better correspondence to muscle isotope values. However, our results clearly demonstrated that isotopic offsets between tissues vary significantly among populations of the same species. Therefore, obtaining a muscle biopsy from several individuals in any population is advisable to gain initial insights and establish a possible population-specific inter-tissue conversion., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Vejřík et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Intra-species variability in isotopic niches, specifically isotopic total niche width (ITNW), isotopic individual niche width (IINW), and isotopic individual specialization (IIS), was studied using an innovative approach without sacrificing the vertebrates. Stable isotopes ( δ 13 C, δ 15 N) in four body tissues differing in isotopic half-life were analyzed from four freshwater fish species representing different trophic positions. ITNW was widest for the apex predator (European catfish) and narrowest for the obligate predator (Northern pike). IINW exhibited a polynomial trend for the European catfish, Northern pike, and Eurasian perch (mesopredator), decreasing with body mass and increasing again after exceeding a certain species-dependent body mass threshold. Thus, for ectotherms, apex predator status is linked rather to its size than to the species. In herbivores (rudd), IINW increased with body mass. The IIS of predators negatively correlated with site trophic state. Therefore, eutrophication can significantly change the foraging behavior of certain species. We assume that the observed trends will occur in other species at similar trophic positions in either aquatic or terrestrial systems. For confirmation, we recommend conducting a similar study on other species in different habitats.

Background: Animal migrations are periodic and relatively predictable events, and their precise timing is essential to the reproductive success. Despite large scientific effort in monitoring animal reproductive phenology, identification of complex environmental cues that determine the timing of reproductive migrations and temporal changes in the size of reproductive aggregations in relation to environmental variables is relatively rare in the current scientific literature., Methods: We tagged and tracked 1702 individuals of asp (Leuciscus aspius), a large minnow species, and monitored with a resolution of one hour the size of their reproductive aggregations (counts of sexes present at the breeding grounds standardized by the sum of individuals in the season) over seven breeding seasons using passive integrated transponder tag systems. We examined the size of reproductive aggregations in relation to environmental cues of day number within a reproductive season (intra-year seasonality), water temperature, discharge, hour in a day (intra-day pattern), temperature difference between water and air, precipitation, atmospheric pressure, wind speed and lunar phase. A generalized additive model integrating evidence from seven breeding seasons and providing typical dynamics of reproductive aggregations was constructed., Results: We demonstrated that all environmental cues considered contributed to the changes in the size of reproductive aggregations during breeding season, and that some effects varied during breeding season. Our model explained approximately 50% of the variability in the data and the effects were sex-dependent (models of the same structure were fitted to each sex separately, so that we effectively stratified on sex). The size of reproductive aggregations increased unimodally in response to day in season, correlated positively with water temperature and wind speed, was highest before and after the full moon, and highest at night (interacting with day in a season). Males responded negatively and females positively to increase in atmospheric pressure., Conclusion: The data demonstrate complex utilization of available environmental cues to time reproductive aggregations in freshwater fish and their interactions during the reproductive season. The study highlights the need to acquire diverse data sets consisting of many environmental cues to achieve high accuracy of interpretation of reproductive timing., (© 2023. The Author(s).)

This article aims to summarize the effects of home training performed during the COVID-19 lockdown on physical performance and perceptual responses among team-sport athletes. Studies with comparison of pre-post lockdown results of physical performance and perceptual responses were considered. A search was made in PubMed and SPORTDiscus databases. The PICO criteria were used for the keywords "athlete" AND "home-based training" AND "performance" OR "mental health", with their respective entry terms. The multistage process of selection followed the PRISMA 2020 recommendations. Of 586 records identified, 9 articles were available for the final process. Physical performance was evaluated for 8 studies with the VO 2max change ranging from 5.7% to -9%; an increase in the duration of sprint test ranging from 0.4% to 36%; an increase of agility duration of 12.4%; a decrease in maximal repetition load of 2.9%; and changes in countermovement jump height ranging from -4.7% to +15.4% after home training. Regarding the perceptual responses, no significant changes in wellbeing and mental index and a significant decrease in motivation and perceived effort were reported during the home training in lockdown. Based on the articles selected, home training programmes performed by athletes from team sports during the COVID-19 lockdown presented inconsistent results in physical performance, decreasing by up to 36%, and maintaining the wellbeing and mental index, but with a significant drop in training motivation and perceived effort. Caution should be taken considering the small number of articles included in the study., Competing Interests: The authors have no conflict of interest relevant to this article., (Copyright © Biology of Sport 2022.)

Background: There is numerous literature on mechanisms underlying variability of practice advantages. Literature includes both behavioral and neuroimaging studies. Unfortunately, no studies are focusing on practice in constant conditions to the best of our knowledge. Hence it is essential to assess possible differences in mechanisms of neuroplasticity between constant vs. variable practice conditions. The primary objectives of the study described in this protocol will be: (1) to determine the brain's structural and functional changes following constant and variable practice conditions in motor learning (structural and functional magnetic resonance imaging, MRI); (2) to determine the EEG activation and connectivity between cognitive, sensory, and motor cerebral cortex areas (central, temporal, parietal, occipital) in constant and variable practice conditions and as a function of practice time., Methods: The study will follow the interventional (experimental) design with two arms (parallel groups). Fifty participants will be randomly assigned to two groups practicing in constant (CG) and variable conditions (VG). CG will be practicing only one pattern of step isometric contractions during unimanual index finger abduction, i.e., 90 trials in all training sessions, whereas VG will practice three different patterns. Each will be practiced 30 times per session in variable conditions. Resting-state fMRI, EEG (cortical networking), and motor task proficiency will be examined before (pre-) and after practice (post- and retentions tests)., Discussion: Findings will enhance our understanding of structural and functional neural changes following practice in constant and variable conditions. Therefore, the study can be considered pure (basic) research (clinical research in healthy individuals)., Clinical Trial Registration: Study registered at clinicaltrials.gov (ID# NCT04921072) on 9 June 2021. Last version update: 21 December 2021.The protocol has been prepared according to the complete SPIRIT checklist (http://www.spirit-statement.org/), although the item order has been modified in order to comply with the manuscript structure., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Czyż, Marusiak, Klobušiaková, Sajdlová and Rektorová.)

The hydropeaking regime below hydropower facilities represents a serious threat to riverine fauna and may cause declines in populations living under its influence. However, the knowledge on direct fish responses to the threat of hydropeaking is limited. Here, we aimed to test whether the hydropeaking generated 12 km upstream may have a negative effect on the position of actively spawning rheophilic fish, asp, Leuciscus aspius. Two passive telemetry antenna arrays were used to record fish position on the spawning ground. We monitored the position of spawning fish (545, 764 and 852 individuals) in three one-month long spawning seasons in 2017-2019 and related the changes in detection probability on the two antenna arrays to flow conditions, temperature, time of a day and individual fish ID. The fish detection on the spawning ground was negatively affected by the flow change (both increase and decrease) in time. Moreover, the probability of fish detection was also influenced by water temperature, the time of the day and, as seen from the magnitude of individual random effect variability, the detection probability was rather individual-specific. Hydropeaking resulted in the change of spawning behaviour and likely caused interruption of spawning or shifting spawning outside the optimal area for egg development. We therefore advise to reduce the hydropeaking regime during the rheophilic fish spawning season under fisheries or conservation interests., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Structural complexity is known to influence prey behaviour, mortality and population structure, but the effects on predators have received less attention. We tested whether contrasting structural complexity in two newly colonised lakes (low structural complexity lake-LSC; high structural complexity-HSC) was associated with contrasting behaviour in an aquatic apex predator, Northern pike (Esox lucius; hereafter pike) present in the lakes. Behaviour of pike was studied with whole-lake acoustic telemetry tracking, supplemented by stable isotope analysis of pike prey utilization and survey fishing data on the prey fish community. Pike displayed increased activity, space use, individual growth as well as behavioural differentiation and spent more time in open waters in the LSC lake. Despite observed differences between lakes, stable isotopes analyses indicated a high dependency on littoral food sources in both lakes. We concluded that pike in the HSC lake displayed a behaviour consistent with a prevalent ambush predation behaviour, whereas the higher activity and larger space use in the LSC lake indicated a transition to more active search behaviour. It could lead to increased prey encounter and cause better growth in the LSC lake. Our study demonstrated how differences in structural complexity mediated prominent changes in the foraging behaviour of an apex predator, which in turn may have effects on the prey community., (© 2021. The Author(s).)

Temporal and spatial heterogeneity in the distribution of cladocerans in lakes could be caused by abiotic (wind, water currents) and biotic factors (reproduction, food resources, predation). Diel horizontal and vertical distribution of cladoceran assemblages was studied in two deep lakes (Milada and Most Lakes, Czech Republic) in early (June) and late (September) summer. The objective was to study diel vertical migration (DVM) and diel horizontal migration (DHM) of cladocerans under conditions of different macrophyte cover in littoral areas (rich in Milada Lake, poor in Most Lake) and fish assemblages (non-specialised planktivorous species in Milada Lake, and abundant planktivorous fish - maraena whitefish, Coregonus maraena - in open water habitats in Most Lake). Temporal variations in cladoceran assemblages were reported in both lakes in the two sampling periods. DVM was observed in the two lakes, performed by the most vulnerable species to fish predation (the larger Daphnia spp.), but with different patterns (direct and reverse) probably linked with the local fish community and other biotic and abiotic factors in each lake. Horizontal movements were only observed in Most Lake: D. longispina increase its abundance in open waters at night compared to the littoral points; while Ceriodaphnia spp. showed the inverse pattern. In both lakes, higher densities were often found at night in surface layers, producing a great "diurnal deficit": cladocerans remain undetected in some zones during the day (especially in the littoral areas) moving to surface layers at night., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Wildlife monitoring using passive telemetry has become a robust method for investigating animal migration. With increased use, this method progressively pollutes the environment with technological waste represented by so called ghost tags (PIT tags ending in the environment due to reproductive expulsions, shedding or animal mortality). However, their presence in the environment may lead to failed detections of living individuals. We used tagging data from studies of the asp Leuciscus aspius and the bleak Alburnus alburnus collected from 2014 to 2018 and located ghost tag positions on the monitored spawning site using portable backpack reader for their detection. We modelled virtual river-wide flat-bed antennas (widths 0.2, 0.4, 0.6 and 0.8 m) representing monitoring effort and estimated the probability of the presence of ghost tags within the antenna field. Of 3724 PIT tags used in the study, we detected on the spawning ground 173 ghost tags originating from long-term monitoring. The ghost tags accumulated in the environment in time, suggesting insufficient degradation rate or shift downstream from the research site. Number of ghost tags present on the spawning ground led to high probability of disabled readings of tagged fish passing through the antenna electro-magnetic field. We demonstrate how accumulated ghost tags may cause detection failures for focal species and incomplete data acquisition. We infer that intensive long-term monitoring using PIT tag technology may encumber future data acquisition or entail additional costs for clean-up., Competing Interests: The authors have declared that no competing interests exist.

Catfish have spread across Europe and several countries out of this region within the last decades. Basic knowledge of this apex predator has revealed concerns of invasive behaviour and questions regarding its utilization as a biomanipulation species. However, a method enabling its regulation to a required level has not yet been developed. We simulated the impact of angling on the catfish population by method of hook-lines in two post-mining lakes with a monitored population consisting of tagged individuals and in two reservoirs as reference sites. Further, the efficiency of hook-lines as a reducing device was examined and the economic aspects were determined. Catfish population in localities where the species is unwanted or invasive may be efficiently reduced to a harmless level by hook-lines and angling (depending on the approach of anglers). The most efficient time of the year seems to be spring to early summer with catch efficiency of 5.4 individuals per 10 baits in one day. The catch efficiency markedly decreased during the second part of the year and did not exceed 2.8 individuals per 10 baits in one day. Mean size of catfish had negative impact whereas catfish biomass had positive impact on the catch efficiency. Trophic status and number of catfish in the locality had no impact on the catch efficiency. According to model, 11-18 bait-days per 1 ha per season is efficient to decrease catfish population to 10% of the original size. Both angling and hook-lines are very simple, they are financially and time bearable mechanisms of catfish regulation in any condition. However, catfish play an important role as a biomanipulative species in many localities. In this case where catfish is beneficial, angling presents a real threat of population collapse and loss of the biomanipulative effect., (Copyright © 2019. Published by Elsevier Ltd.)

Animals that do not provide parental care have to secure the survival of their offspring by ensuring a safe reproductive environment or smart timing tactics. Nocturnal spawning behaviour of many fish species is an example of the latter behaviour in the animal kingdom and is hypothesized to provide a survival advantage to the eggs spawned during the night. In order to test the efficiency of the smart timing tactics in a freshwater fish, a study was carried out of the interaction of the rheophilic spawner (asp Leuciscus aspius) and the predator of its drifting eggs (bleak Alburnus alburnus) using passive telemetry. According to a model based on acquired data, asp laid 63% of its eggs at night, while vision-oriented bleak was present in 92% of the time during the day. This study gives support to the predator avoidance hypothesis, which expects animals to reproduce in a period when the probability of offspring predation is at its lowest.

Two basic ecological relationships, herbivory and competition, distinctively influence terrestrial ecosystem characteristics, such as plant cover, species richness and species composition. We conducted a cage experiment under natural conditions in an aquatic ecosystem to test the impacts of two treatments combined in a factorial manner: (i) a pulse treatment - removal of dominant competitors among primary producers (macroalgae Chara sp. and Vaucheria sp.), and (ii) a press treatment - preventing herbivore (fish, crayfish) access to caged plots. The plots were sampled once before the treatments were established and four more times within two years. Both treatments had a significantly positive impact on macrophyte cover and species richness and changed the macrophyte species composition. The effect of the macroalgae removal was immediate with the highest species richness occurrence during the first post-treatment monitoring, but the positive effect vanished with time. In contrast, preventing herbivore access had a gradual but long-lasting effect and reached a more steady-state over time. Two of the most common species showed contrasting responses, the palatable Potamogeton pectinatus was most supported by caging, while the distasteful Myriophyllum spicatum preferred open plots. Our findings may be applicable during the revitalisation of aquatic ecosystems that aims to increase macrophyte biodiversity.

The higher proportion of males of the invasive round goby Neogobius melanostomus in samples from two activity selective passive fishing gears compared with one activity non-selective fishing gear in three Dutch lakes is related to higher male locomotory activity and is a sex-dependent trait. This difference in activity reflects the different ecology of male and female N. melanostomus., (© 2018 The Fisheries Society of the British Isles.)

The perception of danger represents an essential ability of prey for gaining an informational advantage over their natural enemies. Especially in complex environments or at night, animals strongly rely on chemoreception to avoid predators. The ability to recognize danger by chemical cues and subsequent adaptive responses to predation threats should generally increase prey survival. Recent findings suggest that European catfish ( Silurus glanis ) introduction induce changes in fish community and we tested whether the direction of change can be attributed to differences in chemical cue perception. We tested behavioral response to chemical cues using three species of freshwater fish common in European water: rudd ( Scardinius erythrophthalmus ), roach ( Rutilus rutilus ), and perch ( Perca fluviatilis ). Further, we conducted a prey selectivity experiment to evaluate the prey preferences of the European catfish. Roach exhibited the strongest reaction to chemical cues, rudd decreased use of refuge and perch did not alter any behavior in the experiment. These findings suggest that chemical cue perception might be behind community data change and we encourage collecting more community data of tested prey species before and after European catfish introduction to test the hypothesis. We conclude that used prey species can be used as a model species to verify whether chemical cue perception enhances prey survival.

Apex predators play a key role in ecosystem stability across environments but their numbers in general are decreasing. By contrast, European catfish (Silurus glanis), the European freshwater apex predator, is on the increase. However, studies concerning apex predators in freshwaters are scarce in comparison to those in terrestrial and marine ecosystems. The present study combines stomach content and stable isotope analyses with diet preferences of catfish to reveal its impact on the ecosystem since stocking. Catfish niche width is extremely wide in comparison to the typical model predator, Northern pike (Esox lucius). Catfish and pike have different individual dietary specialization that results in different functional roles in coupling or compartmentalizing distinct food webs. The role of both species in the ecosystem is irreplaceable due to multiple predator effects. The impact of catfish is apparent across the entire aquatic ecosystem, but herbivores are the most affected ecological group. The key feature of catfish, and probably a common feature of apex predators in general, is utilization of several dietary strategies by individuals within a population: long-term generalism or specialization and also short-term specialization. Catfish, similar to other large-bodied apex predators, have two typical features: enormous generalism and adaptability to new prey sources.

For ethical reasons and animal welfare, it is becoming increasingly more important to carry out ecological surveys with a non-invasive approach. Information about fish distribution and abundance in the upper water column is often fundamental. However, this information is extremely hard to obtain using classical hydroacoustic methods. We developed a rigid frame system for pushing upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0-3 m and 3-6 m depth layers, which are utilized by almost all juvenile fish in summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods. The comparison of abundance estimates gathered by the two frequencies were also not significantly different. The predicted mean lengths from acoustic sampling and the trawl catches differed by less than 10 mm in all comparisons. Results suggest that mobile hydroacoustic upward-looking systems can fill the methodological gap in non-invasive surveying of surface fishes.

In polygynandrous mating systems, in which females limit reproductive success, males can increase their success by investing in courtship. Earlier arrival at the spawning ground compared to when females arrive may increase their opportunities in competitive mating systems. In this study, we used passive telemetry to test whether a male minnow known as the asp, Leuciscus aspius, times its arrival at spawning grounds relative to the arrival of females. Males arrived in a model stream approximately five days earlier than females on average and left four to five days later than females over two years. Both sexes performed a daily migration between a staging ground (standing water, low energy costs) and the fluvial spawning ground (high energy costs). Fish abundance peaked twice a day, with a major peak at sunset and a minor peak at sunrise and with the evening peak abundance for males occurring 1 hour 40 minutes earlier than that of females. The number of females on the spawning ground never exceeded the number of males. While the degree of protandry is hypothesized to be influenced by the operational sex ratio (ranging from 0.5 to 1 in our study), our data did not support this theory.

Generalist species commonly have a fundamental role in ecosystems as they can integrate spatially distinct habitats and food-web compartments, as well as control the composition, abundance and behavior of organisms at different trophic levels. Generalist populations typically consist of specialized individuals, but the potential for and hence degree of individual niche variation can be largely determined by habitat complexity. We compared individual niche variation within three generalist fishes between two comparable lakes in the Czech Republic differing in macrophyte cover, i.e. macrophyte-rich Milada and macrophyte-poor Most. We tested the hypothesis that large individual niche variation among generalist fishes is facilitated by the presence of macrophytes, which provides niches and predation shelter for fish and their prey items. Based on results from stable nitrogen (δ15N) and carbon (δ13C) isotopic mixing models, perch (Perca fluviatilis L.) and rudd (Scardinius erythrophthalmus (L.)) showed larger individual variation (i.e., variance) in trophic position in Milada as compared to Most, whereas no significant between-lake differences were observed for roach (Rutilus rutilus (L.)). Contrary to our hypothesis, all the three species showed significantly lower individual variation in the relative reliance on littoral food resources in Milada than in Most. Rudd relied significantly more whereas perch and roach relied less on littoral food resources in Milada than in Most, likely due to prevalent herbivory by rudd and prevalent zooplanktivory by perch and roach in the macrophyte-rich Milada as compared to macrophyte-poor Most. Our study demonstrates how the succession of macrophyte vegetation, via its effects on the physical and biological complexity of the littoral zone and on the availability of small prey fish and zooplankton, can strongly influence individual niche variation among generalist fishes with different ontogenetic trajectories, and hence the overall food-web structures in lake ecosystems.

So far, perch egg strands have been considered unpalatable biological material. However, we repeatedly found egg strands of European perch (Perca fluviatilis) in the diet of European catfish (Silurus glanis) caught by longlines in Milada and Most Lakes, Czech Republic. The finding proves that perch egg strands compose a standard food source for this large freshwater predatory fish. It extends the present knowledge on catfish foraging plasticity, showing it as an even more opportunistic feeder. Utilization of perch egg strands broadens the catfish diet niche width and represents an advantage against other fish predators. Comparison of datasets from extensive gillnet and SCUBA diver sampling campaigns gave the evidence that at least in localities where food sources are limited, multilevel predation by catfish may have an important impact on the perch population., Competing Interests: The authors have declared that no competing interests exist.

The number of herbivores in populations of ectothermic vertebrates decreases with increasing latitude. At higher latitudes, fish consuming plant matter are exclusively omnivorous. We assess whether omnivorous fish readily shift to herbivory or whether animal prey is typically preferred. We address temperature as the key factor causing their absence at higher latitudes and discuss the potential poleward dispersion caused by climate changes. A controlled experiment illustrates that rudd (Scardinius erythrophthalmus) readily utilize plant matter at water temperatures above 20 °C and avoid its consumption below 20 °C. Field data support these results, showing that plant matter dominates rudd diets during the summer and is absent during the spring. Utilizing cellulose requires the enzyme cellulase, which is produced by microorganisms growing at temperatures of 15-42 °C. Water temperatures at higher latitudes do not reach 15 °C year-round; at our latitude of 50°N~150 days/year. Hence, the species richness of omnivorous fish decreases dramatically above 55° latitude. Our results provide support for the hypothesis that strict herbivorous specialists have developed only in the tropics. Temperatures below 15 °C, even for a short time period, inactivate cellulase and cause diet limitations for omnivorous fish. However, we may expect increases in herbivory at higher latitudes caused by climate change.