ABSTRACT The structure of stream networks naturally fragments freshwater habitats, but this complexity is increasingly intensified by dams and reservoirs. How anthropogenic fragmentation interacts with natural isolation gradients to affect fish populations remains poorly understood. Here, we tested how natural (position within the river network, i.e., closeness centrality, and elevation) and anthropogenic (cumulative reservoir volume) isolation gradients influenced body condition of Psalidodon aff. fasciatus across stream reaches distributed in two sub‐basins upstream of Lake Paranoá, Upper Paraná River basin, Brazil. We estimated body condition as residual mass from the length–weight relationship using generalized linear mixed‐effects models, testing effects of isolation predictors and local environmental variables simultaneously. Populations exhibited positive allometric growth, contrasting with typical reports for the species. Contrary to our hypothesis, body mass was significantly higher at sites with greater cumulative reservoir volume downstream. Individuals from the right sub‐basin exhibited greater body mass than those from the left sub‐basin. Natural isolation predictors (closeness centrality and elevation) had no significant effect. Body mass was significantly positively related to water temperature and negatively related to substrate diversity. Our results suggest that body mass varied in response to environmental conditions and the presence of dams, which may benefit this generalist species by expanding foraging habitats and resource availability. These findings highlight that the impacts of fragmentation are species‐specific: while dams can severely affect sensitive species, opportunistic omnivores may exploit newly formed reservoir habitats, potentially reshaping community reassembly in increasingly dammed Neotropical systems.
ABSTRACT Seasonal flood pulses are associated with environmental variability in large river‐floodplain ecosystems, yet the influence on schooling fish remains poorly understood. We used high‐resolution imaging sonar to conduct the first in situ assessment of collective behaviour in a Lower Mississippi River Basin floodplain habitat in order to test the hypothesis that floodplain‐associated schooling fish alter their collective behaviour in response to changes in floodplain connectivity. As schooling is known to be an adaptive strategy, greater behavioural plasticity would allow schooling individuals to appropriately react to changes in environmental conditions, predation risk and foraging opportunities. We recorded 56 h of video across sites located near and far from the initial point of floodplain inundation during connected (high‐water) and disconnected (low‐water) periods spanning all four seasons. School area, alignment (polarisation) and inter‐individual distance (nearest‐neighbour distance) were quantified from 5085 schools comprising 120,114 individuals using a semi‐automated approach. Smallest school areas were observed at near sites during disconnected‐fall. Schools at the far sites during connected‐summer were typically of a larger area than disconnected‐summer at near or far sites. Fish swam in a more aligned fashion during connected‐summer, while inter‐individual distances were lowest during connected‐spring and disconnected‐summer. Results suggest schooling fish inhabiting the floodplain exhibit behavioural changes at the level of their collective in response to changes in the local environmental conditions. Our findings extend the flood pulse concept beyond traditional assemblage‐level response by demonstrating that floodplain connectivity is associated with shifts in the collective behaviour of schooling fishes inhabiting this dynamic environment.
Migratory fish species have experienced substantial declines throughout aquatic habitats, but actions to manage and conserve migratory species require a better understanding of the relative contributions of local populations to the metapopulation. This demographic information is critical for prioritization of conservation and restoration in the most productive spawning tributaries. Our objective was to estimate the contributions from spawning tributaries to Bull Trout rearing in Swan Lake and relate this information to historic redd counts to explore local population productivity and demographics. We used otolith geochemistry and random forest classification to identify spawning stream origins of Bull Trout collected as bycatch in Swan Lake during Lake Trout suppression efforts. Stream assignments for Bull Trout were compared to historic redd count data obtained from surveys over several decades. The results provided evidence that some spawning tributaries contribute disproportionately to Bull Trout in Swan Lake, and streams within the drainage have differential relative production that was not evident by examining redd count data alone. Understanding the demographic dynamics of Bull Trout among spawning tributaries in the Swan River basin will help prioritize effective mitigation actions to aid recovery of this important metapopulation.
Tropical freshwater ecosystems are highly biodiverse and increasingly threatened by anthropogenic pressures, yet the ecological processes structuring their communities remain poorly understood. Here, we examine the effects of hilltop forest conversion to commercially important agro-crop landscapes (tea and cardamom) on fish assemblages in submontane streams of the Sinharaja Forest Reserve, with particular focus on the Critically Endangered, narrow-range endemic loach Schistura madhavai. While changes at the community level were not robustly detectable, species-level responses were more pronounced, with S. madhavai showing a preference for forested streams. Forest modification altered environmental gradients (hydrology, water chemistry, substrate, microclimate, and detritus accumulation), concurrently shifting habitat conditions preferred by the focal species, thereby influencing its occupancy. Associations with environmental variables also differed between ecological levels (e.g., detritus, used here as a food-mediated proxy for benthic insectivores, was more important at the species-level than at the community-level), suggesting that species-specific niche requirements influence the magnitude of responses to disturbance. However, such responses may go unnoticed at the community-level because they are buffered by compensatory dynamics among species and obscured by the coarse resolution of community-level metrics. Thus, environmental filtering may appear weak at broader scales despite strong responses among ecologically specialized taxa. Our findings demonstrate that indicator species can provide a more sensitive measure of biodiversity change than community-level metrics alone. We also highlight the importance of incorporating species-specific ecological requirements into conservation planning, particularly for range-restricted endemics inhabiting threatened tropical freshwater ecosystems.
The mechanisms facilitating coexistence of cryptic species in sympatry have long puzzled ecologists. Cryptic species have nearly identical morphology and, therefore, would be presumed to have high niche overlap and potential for interspecific competition and competitive exclusion when resources are limited. Lowland rivers in South America have hundreds of coexisting fish species, with some congeneric species having similar functional morphology. Here, we document the distributions and compare habitat use of two peacock bass, Cichla cataractae and C. ocellaris, that have nearly identical functional morphology. These piscivorous cichlids are generalist feeders and coexist over a broad region within the Essequibo River Basin in the Guiana Shield region of South America. Data were collected on regional distribution, habitat use and body size to estimate niche overlap. Field research was conducted during the dry season when habitat and food resources are most limited and the potential for competition should be strongest. The two species were found to have negligible overlap in microhabitat use. Cichla cataractae almost exclusively inhabited rocky shoals located in river channels, whereas C. ocellaris occurred mostly within lentic habitats in the floodplain and near-shore areas of channels. This high degree of niche segregation contrasts with the assumption that high morphological similarity should yield high niche overlap and that behavioural and/or physiological differences can be the principal mechanism for niche partitioning.
beta diversity and its decomposed components (replacement and richness differences) reflect the processes shaping the community composition by environmental filtering and dispersal limitation. However, beta diversity does not directly capture species-specific responses to the underlying drivers of community assembly. In this study, we investigated fish communities in the Huishui River and assessed seasonal dynamics of community assembly by integrating analyses of taxonomic and functional beta diversity with species-specific responses to environmental filtering and dispersal limitation. At the basin scale, differences in turnover and richness components exhibited stronger seasonal variation than total beta diversity and responded more distinctly to variations in environmental heterogeneity and community composition. The results from the Hierarchical Modelling of Species Communities (HMSC) indicated that environmental filtering primarily influenced fish community composition in both seasons, with species exhibiting varying responses to environmental and spatial factors depending on the season. These findings suggest that seasonal hydrological variation, together with species-specific differences in environmental responses and dispersal limitation, drives shifts in fish community assembly in this subtropical river. In addition to beta diversity, species-specific responses can offer valuable species-level insights into community assembly, which can be applied for the conservation and management of stream fish.
ABSTRACT Seasonal changes in freshwater ecosystems influence fish resource use through both environmental and biological factors, but year‐round studies that include ice‐covered months have been lacking. We investigated seasonal variation in the stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) using year‐round monthly sampling of the fish community in a deep, seasonally ice‐covered boreal lake in southern Finland. We simultaneously examined how month and a suite of biological variables (total length, sex, gonadosomatic index, condition factor, stomach fullness, C:N ratio of fish muscle) varied across the same period and assessed their relationship with isotope ratios of fish. We found seasonal variations in both δ 13 C and δ 15 N, although the patterns differed among fish species. Three percids: pikeperch ( Sander lucioperca ), perch ( Perca fluviatilis ) and ruffe ( Gymnocephalus cernua ) were enriched in 15 N in the winter, while no change was evident in the cyprinid species. Conversely, roach ( Rutilus rutilus ) and pikeperch 13 C values were enriched in autumn and winter, while no change was detected for perch and ruffe. Total length, month and C:N ratio explained most of the variation in isotope ratios in most species. Results confirm that all four abundant species with year‐round samples of muscle stable isotope ratios are subject to seasonal changes in the studied boreal lake, suggesting that this variation should be taken into account in future spatial and temporal studies.
Increasing numbers of invasive Pacific pink salmon (Oncorhynchus gorbuscha) are spawning in northern Norwegian rivers, leading to large numbers of juveniles migrating to coastal waters. This raises concerns about food competition with native salmonids in rivers, although details about the timing and intensity of pink salmon feeding and the characteristics of their freshwater diet remain unclear. Some evidence indicates that pink salmon can also serve as a food source for native salmonids, as do the carcasses of spawned adults, eggs and juveniles. This study investigated largely unexplored aspects of the emergence timing and feeding behaviour of juvenile pink salmon. The prevalence, timing and impact of fish size on freshwater feeding was examined across 10 rivers in northern Norway, using stomach content and stable isotope (delta 13C and delta 15N) analyses to assess diet composition. Similar methods were applied to assess whether native juvenile salmonids feed on pink salmon tissue, such as juveniles, during their migration. Predictions of the emergence times of alevins show that later spawning, thought to cause increased potential negative interactions with native fish, is an unlikely strategy for pink salmon in northern Norway. In three of the rivers, some of the largest free-swimming pink salmon juveniles sampled in this study (30-35 mm) began external feeding on freshwater invertebrates later in the season (June and July), with larval and pupae chironomids as well as blackfly and mayfly nymphs dominating their diets. Feeding was not observed in all rivers, highlighting the need for further study on the prevalence and intensity of freshwater feeding. Stomach content and stable isotope analyses revealed that some larger native Atlantic salmon and brown trout juveniles consumed pink salmon juveniles, complicating assessments of their overall impact on native salmonids.
ABSTRACT Parasites affect hosts in interaction with the entire ecological community. This is particularly evident for trophically transmitted parasites, such as Dibothriocephalus tapeworms, which infect multiple intermediate fish hosts, potentially reducing their fitness and suitability for human consumption. Here, we used a large, multi‐year dataset to examine ecological underpinnings between Dibothriocephalus burden and host traits and diet. In particular, we assessed relationships between parasite burden and body size, body condition, growth, restocking origin and stomach contents in the key piscivorous salmonids, Arctic charr ( Salvelinus alpinus ) and brown trout ( Salmo trutta ), in a large subarctic lake, Lake Inari. We found that Dibothriocephalus prevalence was similarly high in both salmonids, with decreasing parasite burden over the years. In both species, larger individuals and those having three‐spined ( Gasterosteus aculeatus ) and/or ninespine ( Pungitius pungitius ) stickleback remnants in their stomachs had a higher parasite burden, whereas better body condition, quicker growth, stomach contents other than sticklebacks, stocked origin (in Arctic charr) and river habitat (in brown trout) were associated with lower parasite counts. These findings suggest that Dibothriocephalus burden is likely to be costly, while successful foraging on non‐stickleback food is associated with lower burden. Overall, the results highlight the intricacy of trophic accumulation of Dibothriocephalus parasites, with sticklebacks playing a special role in their transmission.
The Amazon basin is home to one of the richest aquatic ecosystems, playing a key role in maintaining global biodiversity and ecosystem services. Despite extensive studies on fish diversity in the region, gaps remain in understanding broad processes to inform management strategies, such as the socioeconomic aspects of fisheries and the long‐term effects of environmental change on fish populations. We used topic modelling, a machine learning technique, to analyse 400 peer‐reviewed articles on Amazonian fish ecology published over the last 50 years. We identified 13 distinct topics, grouped into five main themes: Ecosystem dynamics, Evolutionary ecology, Hydrological processes, Impact assessments, and Management and Conservation. Results revealed an emphasis on Trophic ecology and Feeding dynamics, Fisheries management, and Evolutionary and Molecular ecology, which make up a large part of the literature. However, topics like Functional extinction and ecosystem consequences and Flood pulse dynamics are less represented, indicating important research gaps. Temporal analysis revealed a shift from early emphases on hydrological processes and participatory management toward contemporary priorities centered on functional ecology, trophic dynamics, and applied management. These findings offer insights for future research and underscore the need for integrated, interdisciplinary approaches to address the region's complex ecological challenges and promote evidence‐based conservation and management strategies.
Biological invasions introduce non-native species into natural ecosystems, often reshaping localcommunities and altering trophic interactions. In freshwater environments, such invasions can intensify resource use and threaten native fish diversity. In the Upper Paran & aacute; River basin, a hotspot with the highest number of threatened fishes in the Neotropics, invasions can be a significant threat. Understanding how non-native fish integrate into local food webs, and how their diets overlap or diverge from those of native species, is essential for assessing the risks they pose. This study investigates dietary patterns of the non-native Poecilia reticulata (guppy) with those of its closely related native species Phalloceros harpagos in streams of southeast Brazil. We analysed the stomach contents of 77 individuals (31 Poecilia reticulata and 46 Phalloceros harpagos) sampled, simultaneously, from five streams where the two species coexist. Dietary patterns were evaluated using a combination of ecological approaches, including trophic overlap (i.e., PerMANOVA, nMDS), niche breadth (i.e., Levins' index) and diet composition (i.e., GLMM). Our results indicate that both species rely heavily on dipterans, particularly Chironomidae, exhibiting low niche breadth and high trophic overlap. However, subtle differences in secondary prey consumption suggest a latent dietary overlap, indicating that minor variations in diet may facilitate coexistence despite the strong similarity in primary resources. This dietary partitioning illustrates how reliance on a dominant prey, coupled with opportunistic consumption of less abundant resources, can facilitate coexistence and stabilise trophic interactions in disturbed freshwater streams.
Seasonal changes in freshwater ecosystems influence fish resource use through both environmental and biological factors, but year-round studies that include ice-covered months have been lacking. We investigated seasonal variation in the stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) using year-round monthly sampling of the fish community in a deep, seasonally ice-covered boreal lake in southern Finland. We simultaneously examined how month and a suite of biological variables (total length, sex, gonadosomatic index, condition factor, stomach fullness, C:N ratio of fish muscle) varied across the same period and assessed their relationship with isotope ratios of fish. We found seasonal variations in both delta C-13 and delta N-15, although the patterns differed among fish species. Three percids: pikeperch (Sander lucioperca), perch (Perca fluviatilis) and ruffe (Gymnocephalus cernua) were enriched in 15N in the winter, while no change was evident in the cyprinid species. Conversely, roach (Rutilus rutilus) and pikeperch C-13 values were enriched in autumn and winter, while no change was detected for perch and ruffe. Total length, month and C:N ratio explained most of the variation in isotope ratios in most species. Results confirm that all four abundant species with year-round samples of muscle stable isotope ratios are subject to seasonal changes in the studied boreal lake, suggesting that this variation should be taken into account in future spatial and temporal studies.
Large wood has been found to be a crucial component in riverine ecosystems. However, due to the heavy channelization of rivers and intensive forestry practices over the past few decades, the amount of large wood in these ecosystems has decreased worldwide. River restoration aims to mitigate the effects of channelization and restore the channel closer to its original, pre-channelized form, but large wood has been an overlooked component in restoration. In this study, we investigated the effect of large wood on the density of brown trout (Salmo trutta L.) parr in several natural rivers in southern Finland and Sweden. The density of 0-year-old parr was higher in areas with a high concentration of large wood, but no difference was found in the densities of 1-year-old or >= 2-year-old parr or other fish species. Other microenvironmental factors did not influence parr density. Due to the apparent positive impact of large wood on 0-year-old parr density, it is recommended as a part of river restoration.
Most salmonid populations are declining across their entire habitat range, partly because of large-scale loss of crucial physical habitats. Alterations in river flow and temperature resulting from climate change are likely to further degrade habitat quality, particularly summer thermal conditions experienced by temperature-sensitive fish species. Understanding how summer thermal conditions control the spatial distribution of ectotherms is thus central to helping project the consequences of climate change and develop management solutions. This study uses snorkelling fish surveys collected over 10 years and airborne thermal infrared (TIR) mapping of surface temperature acquired in 2022 to assess the relationship between European grayling distribution and thermal habitats along a 9-km long reach of the Allondon River, Switzerland. Results show that all 3 grayling life stages (adults, sub-adults and juveniles) respond negatively to elevated summer temperature, with distribution patterns highlighting thermal structuring effects on fish populations. The presence of two cooler reaches appears critical to the survival of the Allondon's declining grayling population, while the warmest reach that separates these habitats potentially acts as a thermal barrier during critical summer conditions. These results were used to guide local stakeholders towards short-term and longer-term actions to be taken on the river, which include: concerted trans-national management to protect key upstream tributaries, tree planting to limit summer peak temperature and strategic protection of cold-water patches that may act as thermal refuges during critical periods.
Migrations are characterised as adaptive mechanisms aimed at improving the fitness of an individual. Hence, the propensity to migrate can be analysed by considering body size and movement activity when individuals search for available resources. We studied the relationships among individual traits, movement activity and migration of the European grayling Thymallus thymallus (Linnaeus 1758) in a native riverine environment during a one-year-long field study. We observed the diel movement activity of 37 individuals along the longitudinal profile of the river using radio tags with temperature sensors. The propensity to migrate differed among the tagged individuals. The fish that maintained positions over the entire annual period and whose dispersal did not exceed 2 km from their original positions were considered a stationary group. The fish migrating downstream from their summer positions to wintering positions and returning to their original positions the following year were considered a migratory group. Long-distance downstream migrations to wintering positions mostly started in early autumn, and the fish reached the farthest positions, up to 30 km apart, in December and January. Upstream migration started at the end of February and was most intensive in March and April. The highest upstream positions were reached by the graylings in summer. These findings demonstrate that fish are motivated to migrate not only to reach spawning areas in spring but also to search for food and shelter resources in summer. Over 24 h periods, the positions along the upstream-downstream axis, movement and body temperature of the migratory and stationary fish differed, indicating their distinct habitat preferences during the diel cycle. Body mass appeared to predict the migration of three-year-old females and males, with a greater migratory propensity for females than males. Our results suggest different behavioural strategies within the population of European grayling, that is, differences between stationary fish and fish migrating long distances among wintering, spawning and feeding areas.
Invasive species pose a major threat to aquatic ecosystems, particularly in high-latitude lakes which are characterised by low biodiversity. In northern Europe, the Eurasian minnow ( Phoxinus phoxinus ) has colonised lakes historically dominated by salmonids, raising concerns about the impacts of invasive cyprinids on native fish populations and food webs. We compared the trophic niche, growth, and maturation of brown trout ( Salmo trutta ) in lakes with and without minnow and assessed dietary overlap between the two species using stomach content and stable isotope analyses. Stable isotope analysis revealed that in lakes with minnow, trout exhibited more pronounced ontogenetic niche shifts from pelagic to littoral feeding and towards higher trophic positions compared to lakes with only trout. The isotope data also showed that small trout overlapped in trophic niche with minnows. Stomach content analysis revealed a shift in trout prey use, with reduced consumption of Eurycercus lamellatus and Gammarus lacustris , increased use of surface insects and a transition towards partial piscivory (prevalence of piscivory 5.5%). Despite potential resource competition at early life stages and shifts in diet, when coexisting with minnow, trout grew faster and females showed a tendency to mature earlier. Overall, the presence of invasive minnow does not appear to negatively affect native trout. This is likely due to a combination of flexible resource use and the opportunistic piscivory exhibited by trout. Since our study systems were recently invaded, the findings provide new insights into how native salmonids respond to invasive species shortly after their establishment in small high-latitude lakes.
In recent years, small-bodied fishes have become increasingly dominant in many subtropical reservoirs due to climate change and overfishing of predatory species. Their rising populations can disrupt aquatic ecosystem balance and increase eutrophication risk. This study investigated the density-dependent effects on the reproductive traits of two dominant small-bodied fish species (sharpbelly Hemiculter leucisculus and thin sharpbelly Toxabramis swinhonis), and their subsequent impacts on plankton biomass in three large Chinese reservoirs during 2022-2023. We observed significant variation in reproductive traits of the two fish species; both species exhibited the lowest fecundity, largest egg diameter, and smallest length at 50% maturity in the Liuxihe Reservoir, which had the highest density of thin sharpbelly. Density-dependent responses differed between species: sharpbelly fecundity was primarily affected by interspecific competition with thin sharpbelly, whereas thin sharpbelly fecundity was mainly influenced by intraspecific competition. Both species affected realised zooplankton biomass (as indicated by the ratio of zooplankton biomass to phytoplankton biomass) and positively promoted realised phytoplankton biomass (as indicated by the ratio of chlorophyll a to total phosphorus content). These results suggest that both species exert top-down control on zooplankton, which indirectly promote phytoplankton growth and negatively affect water quality. Our findings highlight that the reproductive traits of these small-bodied fish species were closely linked to species-specific densities, which in turn drove top-down impacts within freshwater food webs. These findings underscore the ecological risks posed by high densities of small zooplanktivorous fish and emphasise the importance for population regulation in freshwater ecosystems management.
The critically endangered European eel (Anguilla anguilla) faces major threats from hydropower turbines during downstream migration, which is a critical phase in the species' life history. To mitigate impacts on eel, engineering solutions can be implemented that enhance passage success around turbines. This study assessed the effectiveness of retrofitting the intake trash rack at Fosstveit Hydropower Plant in the River Storelva, Norway, to improve safe passage for silver eels. In 2012 and 2013, PIT-tagged eels showed high turbine mortality, with survival rates of 52% and 69%, and dead eels were observed in the tailrace. A conventional trash rack with a bottom bypass was evidently ineffective during these years. In 2024, the intake was retrofitted with a 15 mm bar spacing, low-inclined (29 degrees alpha) rack guiding fish to a surface bypass. In the following years, 2024 and 2025, acoustically tagged eels released upstream the facility showed 100% survival to the river mouth. Most eels migrated during the dark hours and primarily on days with high or rising discharge, consistent with known cues. Although migration past the hydropower plant remained slower than in downstream sections, the retrofit greatly improved survival. These results support the use of inclined low bar spacing rack and surface bypasses at hydropower plants and underscore the need to address migration delays and explore adaptive flow management to support eel recovery.
The frequency, magnitude and extent of stream drying is increasing due to climate change and human water demand. Fish vulnerability to increased stream drying is a combination of sensitivity (innate tolerance to low streamflow) and exposure to stream drying. To understand fish tolerance to low flow and susceptibility to decline under changing streamflow conditions, we estimated species-specific measures of sensitivity to low streamflow, determined relationships to species traits and evaluated vulnerability to future reductions in streamflow for 60 species. We found that sensitivity varied across species, and some variation was explained by life history strategy, spawning strategy and body size. Periodic life history strategy, pelagic spawning and larger size corresponded to an increased sensitivity to stream drying. Under future projections of August streamflow, 90% of sites were predicted to decrease in flow magnitude. Vulnerability to changes in streamflow, the combination of sensitivity and exposure, varied slightly across the study species, with the percent of inhospitable sites under future climate scenarios increasing for 87% of the species. Despite being relatively insensitive to low streamflow, vulnerability was high for multiple species dominant in mountainous areas, driven by high levels of exposure to stream drying. Our results illustrate the potential for species traits to predict sensitivity to low streamflow and demonstrate that exposure may play a large role when defining species vulnerability to stream drying. The ability to predict species tolerances and susceptibility to decline will become increasingly important in prioritising conservation efforts under changing environmental conditions.
