The size spectrum, which describes the relationship between abundance (or biomass) and body size, is an ataxic approach that can provide insights into energy fluxes across trophic levels. However, anthropogenic perturbations can alter the relationship between body size and trophic position, and therefore, the predator-prey mass ratio (PPMR). In this study, we used body size distribution and stable isotope analyses to investigate the relationship between size spectrum and the PPMR in lake fish communities across various eutrophication and invasion levels. Our results revealed that, although size spectrum and PPMR co-varied (i.e. resulting in flatter size spectrum when PPMR was low), this effect was modulated by the level of biological invasion in the community. This was likely caused by differences in trophic niche between native and non-native species: small non-native species exhibited higher trophic positions than small native species, while large non-native species can have lower trophic positions than their native counterparts. These findings suggest that the relationship between size structure and trophic interactions in lake fish communities may be blurred by anthropogenic perturbations, challenging core assumptions of size-based ecology in estimating energy fluxes within freshwater food webs.

Understanding intraspecific variation in upper thermal tolerance is essential for assessing climate change impacts on freshwater fish communities. We experimentally evaluated thermal tolerance in a cyprinid fish, Pseudaspius sachalinensis, using critical thermal maximum (CTmax) measurements. Fish were collected from two tributaries in Hokkaido, Japan, differing in thermal variability but with similar mean temperatures. The population experiencing greater thermal fluctuations exhibited significantly higher CTmax, whereas individual-level factors (body size and condition) had no detectable effect. These findings highlight the importance of population-specific conservation strategies under climate change.

Abstract Parasites can profoundly alter host communities. However, the impact of parasites can vary from one community to another. Understanding why the impact of parasites varies across communities is challenging because it requires (i) separating the direct effects of the parasite on the host species from the indirect effects it exerts through the ecological interactions among the hosts and (ii) identifying how the presence of one host alters rates of infection in another. Freshwater fish communities in Trinidad have recently been invaded by a generalist parasitic nematode. This event, combined with our long‐term mark‐recapture studies of these communities, which began long before the invasion, presented a replicated natural experiment. In this experiment, we measured how host demographic rates responded to two ‘treatments’: stage of parasite establishment (before, and 1–2 years, 3–5 years and 6–8 years after invasion) and type of community (killifish‐guppy communities and killifish‐only communities). This design allowed us to infer the direct and indirect impacts of the parasite invasion on killifish communities and determine how the presence of guppies altered infection rates in killifish. The parasitic invasion drastically altered killifish‐guppy communities: the ratio of killifish to guppies changed from 1:2 before the invasion to 1:16 after the invasion. Living with guppies amplified the effects of the parasite on killifish, such that parasite‐related mortality rates of large adult killifish were twice as high in communities with guppies compared to those without. This effect was driven by a doubling of infection rates in large killifish that live with guppies. In a parallel study, we identified the same patterns of size‐ and community‐dependent infection rates of killifish in five separate river systems, implying that this pattern is general in this system. Our study provides mechanistic insight into how parasites alter community structure under natural conditions, via their direct and indirect impacts on host demographic rates. Our work highlights the value of long‐term field studies for our understanding of the impact of parasites on community structure and of ecological interactions in general.

Various anthropogenic disturbances affect the succession of aquatic habitats along dendritic river networks. Bioindicator taxa, such as fish, can be used to assess the effects of these disturbances on habitat quality. Environmental DNA (eDNA) metabarcoding offers a novel approach to complement traditional sampling and analysis of bioindicator taxa. Here, we apply a trait-based biomonitoring framework, focusing on fish tolerance to pollution, to assess habitat quality and fragmentation within two watersheds in southern Québec (Canada). We sampled 193 sites within the dendritic networks of the Châteauguay and St. François watersheds and estimated fish community tolerance indices on the basis of 12S metabarcoding. We found a significant correlation between the fish community tolerance index and environmental factors such as subwatershed land use, precipitation and elevation. We also found that river fragmentation caused by dams affected fish assemblages and native fish movement but also prevented the spread of the non-native common carp. Finally, we applied random-forest modelling to predict the tolerance of fish communities to disturbances in unsampled areas, providing a broader understanding of habitat quality within catchments. Our research highlights how eDNA metabarcoding for large-scale biomonitoring and river fragmentation studies provides a cost-effective and non-invasive method for assessing fish biodiversity and riverine ecosystem health.

ABSTRACTUnderstanding the intricate dynamics of biodiversity within and across riverine ecosystems, influenced by geological history and environmental factors, is crucial for effective conservation and management strategies. Italy, particularly the Ligurian region, harbors diverse freshwater fish communities and populations shaped by unique geological and hydrological conditions. Here, we investigated the suitability of environmental DNA (eDNA) metabarcoding to identify inter‐ and intraspecific diversity patterns of riverine fish populations in drainage basins on both sides of the main drainage divide (MDD) between the Adriatic and Ligurian river basins in Northern Italy. We collected 96 aquatic eDNA samples across 48 riverine sites, amplified them using a cytochrome b primer pair, and denoised the sequences to retrieve amplicon sequence variants (ASVs). We calculated communities' phylogenetic distance with betaMPD based on genetic distances derived from the ASVs, combined them with conductance‐based landscape metrics, and applied generalized dissimilarity models to assess spatial genetic structure. Our results reveal genetic differentiation among populations of several fish species, with some displaying clustering patterns across the main drainage divide and isolation by distance patterns. Overall, taxon richness was higher on the Adriatic side (12.82 ± 3.57; 26 unique taxa) than on the Ligurian side of the MDD (8.35 ± 3.66 SD, 25 unique taxa), but the other way around for ASV richness across all species (Ligurian side: 51.94 ± 25.79, 308 unique ASV, Adriatic side: 68.00 ± 32.97, 274 unique ASV). Our findings highlight the effectiveness of eDNA metabarcoding in uncovering various facets of diversity, shedding light on hidden genetic diversity among ASVs, and revealing significant spatial genetic structuring in freshwater fish populations across multiple species.

Shoreline alteration from human development and armouring is a major driver of biodiversity loss in freshwater ecosystems. While biologically inspired shoreline armouring has been explored in marine systems, its application in freshwater remains limited. This study advances freshwater research by moving from lab-based choice experiments to field trials, allowing fish to interact freely with textured panels under natural conditions. We compared fish community assemblages across four shoreline treatments: natural, textured panel, flat panel, and altered shorelines. Fish abundance, species richness, and diversity indices were assessed using snorkelling surveys over a single summer season. Species-specific patterns emerged. Bluegill and pumpkinseed were most abundant across all treatments, whereas textured panels supported higher abundances of largemouth and smallmouth bass, particularly young-of-the-year individuals. Age-class composition and proximity distributions differed significantly among treatments for several species, indicating treatment-specific habitat use rather than changes in overall diversity. These findings suggest shoreline modification influences fish community composition and spatial use more strongly than overall abundance or richness, especially within largely natural landscapes. Despite their limited size and short deployment, textured panels may provide functional habitat for select species, highlighting the importance of species-specific responses in evaluating eco-engineered shoreline designs.

Global change reshapes biodiversity through shifts in species composition, richness, and body size. How these shifts combine to alter higher-level ecological processes within food webs can have important implications for entire ecosystems. However, the strength and direction of these shifts will depend on combinations of ways that species and trait compositions change through time. We combine long-term data from ~15,000 freshwater and marine fish communities (1949-2019) with information about their size, diets, and trophic status to evaluate how food webs change through time at local spatial scale. We found that selective species turnover driven by body size reductions is associated with widespread alteration to fish food web topology and function, including increased connectance and generalism, leading to higher predation pressure and increased prey vulnerability. Food webs were also less modular. These changes extend across food web trophic structures, causing a cascading shift in the proportion of species across trophic levels. Our study highlights complex biodiversity responses to confluent changes across multiple facets.

Accelerating collapse of freshwater ecosystems and fish communities in North Africa's Middle Atlas under combined climatic and anthropogenic pressures

Endemic species enrich biodiversity hotspots, but how do they contribute to biodiversity structure at macroecological scales? Here, we argue that classifying endemic species using a framework defined by the complementary axes of taxonomic and functional diversity is key to revealing how these patterns underpin community convergence and divergence. These processes are known to configure communities to be compositionally more similar or dissimilar, respectively. Using the endemic freshwater fish communities of India's Western Ghats Escarpment (WGE), one of the 'hottest' spots of global biodiversity, as a test case, we find that geographically widespread, trait-distinct endemics are disproportionately present in the west-flowing basins of the WGE where they promote overall convergence (i.e. both taxonomic and functional). In contrast, among east-flowing basins, a lower-than-expected occurrence of the same category of species supports taxonomic divergence and functional convergence. We attribute this heterogeneity to western-flowing basins having higher (i) ecosystem productivity that supports trait-distinctiveness, and (ii) temporary lateral connectivity that facilitates fish dispersal. Our study demonstrates how different dimensions of diversity interact to produce ecological structure, thereby underlining their role in resilience. Thus, this framework has application in conservation and policy and can guide global efforts to protect endemic biodiversity in hotspots.

ABSTRACT Seasonal migrations of diadromous fish transport marine‐derived nutrients into freshwater ecosystems, enriching communities and restructuring food‐web dynamics. By diminishing these migrations, fishing pressure could disrupt cross‐ecosystem connectivity, thereby altering predator–prey interactions and community size structure. Despite this potential for fishing‐induced disruption, the functional role of migratory species remains poorly understood. We examined whether fishing‐induced declines in amphidromous post‐larvae, or ‘whitebait’, from three galaxiid species ( Galaxias fasciatus , G. argenteus and G. postvectis ) influenced freshwater fish community size structure and enrichment by reducing a potential marine‐derived prey subsidy. We compared fish community diets (using stable isotope analyses) and size structure within five fished and three unfished streams on the West Coast of New Zealand's South Island before and during peak whitebait migration. Whitebait prey were primarily consumed by large (> 100 g) piscivorous fish, particularly longfin eels ( Anguilla dieffenbachii ) and, to a lesser extent, G. argenteus . Despite substantially greater whitebait influxes in unfished streams, there was no difference in the abundance of large fish between stream types or migration periods. However, abundances of medium‐sized (10–100 g) fish increased during peak whitebait influxes, especially in unfished streams. Although no size‐specific differences in whitebait consumption were observed between stream types at the individual level within any species, the greater whitebait influx in unfished streams likely supported a greater number of piscivorous fish, explaining the increase in medium‐sized fish abundance. Moreover, despite generally not appearing to consume whitebait, G. postvectis diets were dominated by freshwater prey in unfished streams and by a combination of terrestrial and freshwater prey in fished streams, suggesting indirect effects of whitebait influxes on non‐piscivorous fish. Overall, although predominantly consumed by large piscivorous fish, influxes of marine‐enriched whitebait influenced recipient ecosystems by directly and indirectly shifting predator–prey interactions and alleviating food limitation for subordinate fishes. Incorporating these measures within an ecosystem‐based framework that accounts for predator–prey interactions and intraspecific and interspecific competition will help safeguard aquatic ecosystem integrity and support both migratory and resident fish populations over the long term.

Catastrophic mortality occurred in the fish community at Serayu Movable Dam in 2022. The phenomenon was caused by severe water quality alteration due to sediment flushing from Panglima Besar Soedirman Dam. After the incident, the fish community in the reservoir has never been evaluated. This study aimed to assess the fish community status in the Serayu Movable Dam post-catastrophic mortality in 2022. The study was conducted using a survey method, and fish samples were collected three times at six sampling sites at an interval of two weeks. Fish identification was done using the identification key in the references. Various community and water quality parameters were measured. The data obtained were analyzed descriptively by comparing them with standards in the literature. A total of 27 species were identified with a richness index of 4.564, relative abundance of 0.037, diversity index of 2.111, evenness index of 0.693, and dominance index of 0.259. The data proved that the fish community in Serayu Movable Dam has recovered after two years of catastrophic mortality in 2022. It could be due to the characteristic of the riverine ecosystem that can revert due to the continuous replacement of water from various tributaries that flow into the Serayu River in the upstream part of the Serayu movable dam, as shown by good water quality. It can be concluded that high ichthyofauna diversity was observed in the Serayu Moveable Dam post-catastrophic mortality. The data are essential scientific bases for sediment flushing management at P.B. Soedirman Dam and management of freshwater fish community in Serayu Movable Dam.

Season and Environmental Factors Contribute to the Variation in Functional Diversity of Freshwater Fish Communities in Floodplain Wetlands of Assam, Northeast India

ABSTRACT According to the European Water Framework Directive 2000/60/EC the fish fauna was investigated in 16 sites belonging to 12 Sicilian watercourses, several of which were within Sites of Community Importance and/or regional reserves and/or parks. This activity had four objectives: 1) to provide for the first time an evaluation of the ecological status of Sicilian rivers through ISECI, the fish index approved by the Italian Ministry of the Environment. 2) to compare the performance of ISECI with its updated form NISECI, which is now the reference Italian index 3) to first evaluate the reference freshwater fish communities provided by the official method approved by the Italian Ministry 4) to compare the fish indexes to other biological indexes. Five fish species belonging to three families were found: two native to Sicily, two non-native and one parauthocthonous (i.e.: introduced and established since very ancient times). The values of the two indices calculated based on the reference communities outlined in the official methodology overall showed a significantly critical situation, as no site was classified as Good or High. The study highlighted several problems concerning their application in a Mediterranean, freshwater species-poor island, the main of which was the discrepancy between the reference fish community reported in the official method and the communities really present in the island. A simulation done with “refined” reference communities revealed some differences between the two indexes. Some proposal of improvement have been outlined, like: verify, review and refine the reference fish communities; expanding the fish monitoring network; experimentation of innovative methods.

Invasive alien fish species, such as the largemouth bass, Micropterus salmoides (Lacepède, 1802), and bluegill, Lepomis macrochirus Rafinesque, 1819, have rapidly spread across freshwater ecosystems in Korea, raising serious ecological threats. This study investigated the impact of these invasive alien species on fish communities in large lakes and small reservoirs over a 5-year monitoring period (2019–2023). Eight sites, including four lakes and four reservoirs, were surveyed twice a year. Fish were collected using cast nets, kick nets, and gillnets (for lakes). Species diversity indices (Shannon’s Hʹ, Pielou’s Jʹ, and Margalef richness) and dominance indices were calculated. Poisson-based analysis of variance models was applied to examine the relationship between the densities of invasive alien species and the diversity of fish communities. Sites with higher densities of largemouth bass and bluegill had significantly lower diversity indices. Non-metric multidimensional scaling analyses indicated distinct community groupings according to the type of waterbody and the presence of invasive alien species. These results emphasize the need for continued monitoring and management strategies tailored to different habitat scales.

Anthropogenic barriers fragment Portuguese rivers, threatening endemic freshwater fish communities. This study compiled national inventories and peer-reviewed research (2002–2024) to quantify fishway implementation, evolution and typology, while evaluating fish performance from published research. One hundred fishways built between 1950 and 2024 were recorded, half of which were constructed after the implementation of the Water Framework Directive in Portugal (29 Dec 2005), tripling the annual construction rate. Fishways were found to be associated mainly with weirs (46%) and small hydropower plants (44%), with typology being dominated by the pool-type design (67%), nature-like facilities (18%), fish locks and combined systems (6% each), fish lifts (2%) and a single eel pass. Forty scientific contributions addressed fishway effectiveness; three-quarters dealt with pool-type facilities, while 12.5% and 10% focused on nature-like fishways and lifts, respectively. Experimental and field studies highlighted species-specific hydraulic preferences, the benefits of vertical slot and multislot configurations, and the potential of retrofitting fishways with macro-rugosities (i.e., fixed structural elements placed on the bottom) to improve non-salmonid fish passage. However, low attraction efficiency, limited multi-season monitoring and risks of aiding invasive species remain a concern. Research needs are proposed, including the refinement of species-specific hydrodynamic criteria, and the development of standardized efficiency metrics and of selective passage solutions, to advance fishway performance under Mediterranean hydrological constraints.

Biological communities are facing profound upheaval induced by global environmental change. While changes in community composition over time are now well documented, much less is known about whether concomitant shifts in trophic structure also manifest. Here, we leveraged a 10‐year dataset of freshwater fish communities and stable isotope compositions in nine lakes to test whether compositional changes (i.e., changes in community structure) and local environmental factors drove trophic trajectories over time. We found marked changes in the trophic structure of fish communities across all lakes, with a general tendency toward narrower trophic niches dominated by trophically redundant species. The variations in trophic trajectories among lakes were primarily linked to differences in the temporal pace and directionality of change. Specifically, lakes exhibiting greater compositional changes displayed more irregularity in their trajectory, and communities dominated by non‐native species displayed elevated trophic stability over time. Our findings reveal species turnover as the dominant factor shaping trophic dynamics, through the addition or removal of predatory species and trophic turnover. The trophic stability observed in communities that were already invaded at the start of the study could be driven by their reduced susceptibility to compositional change caused by subsequent invasions. These findings highlight the existence of strong changes in trophic niches and unveil the intricate interplay between compositional changes and biological invasions in governing the trophic trajectories of communities and food web architecture, with subsequent implications for ecosystem functioning.

ABSTRACT Freshwater fish communities differ widely across spatial scales due to environmental filtering and limited dispersal. However, the relative importance of these mechanisms is poorly understood, especially along subtropical elevational gradients. Species traits may further help to predict species niches and co‐occurrences and allow for generalisations across multiple ecosystems. We aimed to model the occurrence of fish species as a function of environmental covariates and distance, testing for the effects of traits and phylogeny on species niches and co‐occurrence patterns. We also aimed to test the consistency in these relationships between the river basins. We fitted two joint species distribution models (JSDM) to a dataset of fish sampled over multiple years, across the Teesta–Jaldhaka basins of the Eastern Himalayas. The effects of environmental filtering were stronger than those of distance. Elevation, total dissolved solutes, dissolved oxygen and stream width explained much variation in species occurrences, but their relative importance differed among basins. Traits explained relatively less variation in species niches and co‐occurrences, and their effects differed among basins. However, the residual phylogenetic signal in niches indicates that unmeasured traits likely play a stronger role. Environmental filtering primarily structures lotic fish communities along subtropical elevational gradients in the two adjacent Eastern Himalayan River basins. However, the relative influence of covariates and the nature of trait–niche relationships differs. This variability makes broad generalisations difficult across freshwater basins while providing an opportunity to test and explore the applicability of ecological filters in other riverine systems.

(1) Background: Affected by multiple factors, the decline in fish species diversity in some aquatic ecosystems has become increasingly pronounced. At a broad spatial scale, economic development has been widely recognized as one of the key factors influencing the fish distribution pattern. However, at a small scale, within a single river basin, the effects of economic development on the freshwater fish distribution and communities remain largely uninvestigated. (2) Methods: environmental DNA (eDNA) samples were collected from 26 sampling sites of the Lixian River in both the summer (June) and winter (November). Economic data from the Lixian River basin were collected, and analyses, including multivariate regression tree analysis and generalized linear model fitting, were performed using R 4.3.2. (3) Results: A total of 65 fish species was characterized, and the Chao1 diversity indices in the upstream (13.42) and downstream (13.00) were significantly higher than those in the middle reaches (8.55, p < 0.01) of this river. The species communities exhibited an obvious gradient changing pattern from the upstream to the downstream reaches, with parameters of water quality, including transparency, pH, dissolved oxygen and temperature, and climatic factors functioning as the key variables. Furthermore, the generalized linear model analysis revealed significant positive correlations between agricultural population (p = 0.00106), total grain production (p = 0.00476), total population (p = 0.00192) and the Chao1 index. (4) Conclusions: Climatic factors are the key factors affecting the fish diversity in the Lixian River. In less economically developed areas, the development of local economic activities may enhance fish diversity.

Biological invasions by non-native fish species pose a major threat to global freshwater ecosystems. However, our understanding of why invaders establish in some communities but not others (i.e., occurrence probability) and why some communities harbor more invaders than others (i.e., invasion richness) remains limited. Here, we combine species- and community-level analyses across taxonomic, phylogenetic, and functional dimensions to investigate both the occurrence probability and invasion richness of exotic and translocated fish species across nearly 3000 North American freshwater fish communities. We found that ecological similarity between non-native and native species primarily governed non-native fish establishment, with invaders more likely to succeed when closely related to native species. In contrast, invasion richness was largely promoted by human pressure, while native diversity reduced richness specifically for translocated species. Additionally, both the occurrence and richness of invaders increased toward lower latitudes. Together, these findings highlight the distinct yet complementary roles of environmental filtering, human activities, and biotic interactions in shaping freshwater fish invasions.

Domestic and exotic freshwater fish communities of different water systems under anthropogenic pressures: Diversity patterns, environmental drivers and ecological interactions.