ABSTRACT 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.

ABSTRACT 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.

ABSTRACT Identifying the mechanisms that underlie how organisms respond to current and expected hydrologic alterations is essential to understanding the effects of climate change on riverine systems. Life history theory is one such mechanism and forms clear predictions about how traits (e.g., body size) or specific groups of traits (strategies) will be selected for by hydrological conditions. However, few researchers have tested hypotheses informed by life history theory across significant spatial and temporal scales simultaneously, or considered how selection acts on all traits or specific strategies. Our goal was to use a stream fish community dataset spanning five river basins and over two decades in the National Forests of Mississippi to test the hypothesis that (1) life history traits would be selected for by environmental conditions, and (2) that life history strategies would be selected for by hydrological conditions. To test the first hypothesis, we used a permutation procedure to compare functional beta diversity between observed and randomised datasets. To test the second hypothesis across life history strategies, we first fit beta‐binomial models with life history strategy (identified through archetype analysis) as the dependent variable and predictors of streamflow as independent variables, and then identified interpretable models using model selection. Our results did not support the first hypothesis or the second hypothesis within opportunistic strategies. However, we did find evidence that equilibrium strategists were selected for by less variable hydrology. Our results fill an important data gap within the southeastern U.S. and suggest that the influence of life history traits on community structuring is not as influential within our study system as within previous studies. We hypothesise that this could be due to the lack of environmental extremes within the dataset. Future studies should expand on the current study by investigating how selection may act differentially on individual traits and life history strategies.

ABSTRACT Nest association is a putatively mutualistic mode of reproduction utilized by many fishes of the North American freshwater family Leuciscidae. It is defined by the interaction between one ‘host’ species that builds and maintains a nest, and multiple other species of ‘nest associates’ that spawn upon it. We studied the aggregations of spawning Nocomis leptocephalus in order to identify potential embryo predation behaviors of the host or associates. We investigated the effectiveness of host parental care by comparing predation upon buried and unburied embryos. The main observed parental care behavior of the host—embryo burying—increased embryo survival by 26.8 (±5.6%) on average, indicating that it is an effective method of improving reproductive success. We hypothesized that both host and associates are responsible in part for embryo predation, and that nest associate abundance increased the risk of embryo predation. DNA metabarcoding of gut contents revealed that indeed both host and associates consumed heterospecific embryonic material, whereas model‐based evidence suggested that associate abundance only weakly affected embryo predation rate. This work furthers and supports previous research recognizing the nuance in identifying nest association as a mutualism, and work which has suggested the host benefits from nest associates beyond the predation‐dilution effect on its embryos. Here we identify an additional benefit to hosts and associates—the direct consumption of each other's embryos—making this the first study to directly investigate and demonstrate predation upon embryos by nesting fishes in this system.

ABSTRACT Freshwater habitats in the Indo‐Pacific Islands are colonised by amphidromous fish, which undergo a marine larval phase while the rest of the life cycle takes place in the riverine habitat. These fish are mainly represented by Gobiiformes. Among these, Stenogobius genivittatus (Valenciennes, 1837) is widely distributed in the Indo‐Pacific Region, from South Africa to French Polynesia. We studied elements of the life history of S. genivittatus to better understand the processes contributing to its vast distribution: we analysed population genetic structure based on partial mitochondrial DNA sequences ( COI and ND6 ) and we used otolith microstructural analysis to estimate the duration of the marine pelagic larval phase (PLD). The analysis indicates the presence of a unique population broadly distributed across the Indo‐Pacific Ocean, from South Africa to French Polynesia, while individuals from Hawaii appear to constitute a potentially distinct population. Also, our results show a homogeneous PLD in the West Indo‐Pacific Region, except for Hawaii where it is longer and the Central Pacific Ocean (French Polynesia and the Marquesas) where it is shorter.

ABSTRACT Models describing associations between fish distribution and environmental or spatial gradients at the population level have the potential to be transferrable if model parameters are stationary among populations and over years. Further, population‐level habitat association models represent the scale of effect—habitat relevant to within‐population distribution and processes. Here we show that for a widely recognised warm water fish species (smallmouth bass; Micropterus dolomieu Lacepède, 1802), habitat use extends into the metalimnion and hypolimnion of lakes. Lake depth at net sites and temperature at capture depth were used to model habitat use in a multi‐lake set ( n = 11 lakes) and for a large lake with three surveys over a decade. In the multi‐lake set, a depth model was top ranked with little difference among lakes. In the lake with multiple surveys over a decade, a quadratic temperature model was top ranked but resulted in among‐year differences in occupancy levels at any given temperature. The second ranked depth model produced consistent occupancy patterns with depth and matched parameter values from the multi‐lake model. This consistency points to a general habitat association model based on depth for smallmouth bass during the summer season. We provide guidance for habitat managers based on this stationarity.

ABSTRACT 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.

ABSTRACT 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.