River Fish Community Research Articles

Freshwater habitat invaders: Impacts on artificial flow refuge use by native fish under pulsed flows

Flow regulation and the spread of invasive species are significant threats to freshwater biodiversity, including native fish communities in rivers across the globe. The bleak (Alburnus alburnus), an invasive fish currently widespread throughout the main rivers of the Iberian Peninsula, threatens native fish biodiversity, mostly via trophic competition and hybridisation. In this study, we assessed the impacts of bleak presence and rapid flow fluctuations (both main and interactive effects) on the use of artificial habitats (a flow refuge) and the stress physiology of barbels. We examined how bleak's presence influenced the selection of flow refuges by the native Iberian barbel (Luciobarbus bocagei) under base (continuous 7 L s−1 flow stimulus) and pulsed (7 L s−1 to a 60 L s−1 pulse) flows in an indoor flume. We quantified habitat use within three specific regions of two installed flow refuges and two physiological responses (glucose and lactate) associated with the stress axis. The results indicated that the presence of bleak significantly reduced flow refuge use by barbels and this effect was more prominent during pulsed flows. Additionally, barbels exhibited higher levels of glucose and lactate in the presence of bleaks. These results show that bleaks compete with barbels for habitat under fluctuating flows and induce physiological adjustments that may reduce the ability of barbels to find low-velocity areas to shelter. Overall, the bleaks had a competitive advantage under high flow variability. This underscores the need for further studies and effective management strategies to protect native freshwater fish communities in the context of biological invasions and anthropogenic flow variability.

Functional turnover in a prairie river fish community over 130 years

AbstractObjectiveIn many Great Plains rivers, functional turnover—the change in proportional dominance of members in biological communities that fill certain ecological roles—has occurred due to impoundment and habitat alteration. The Powder River of Montana and Wyoming remains one of the few unregulated prairie rivers, but long‐term monitoring is limited, so we analyzed changes over time at the functional, assemblage, and species levels.MethodsWe used fish sampling data from 43 different sources collected from 1893 to 2022 to analyze trends in fish communities.ResultAcross the main‐stem Powder River, Sand Shiner Miniellus stramineus and Channel Catfish Ictalurus punctatus substantially increased in abundance, whereas Sturgeon Chub Macrhybopsis gelida decreased. While most other species did not show significant changes in relative abundance (although the always rare Lake Chub Couesius plumbeus may have been extirpated), significant functional turnover occurred in the upper river due to increases in generalist feeders, predators, omnivores, and cavity‐guarding species, with declines in benthic feeders, invertivores, and pelagic broadcast spawners, among others. Community and functional changes were more substantial in the upper river than in the lower river, possibly due to augmented streamflow from a major tributary.ConclusionFunctional turnover within the upper river was substantial despite the relative stability of most individual species, even when the Sand Shiner—the most significantly increasing species—was excluded from analysis. This suggests small but consistent increases and decreases within functional groups, which cumulatively are likely impacting the ecosystem. We hypothesize a complex set of mechanisms causing these changes that offer avenues for future work. The collation of data from disparate studies and the resampling of even a limited number of historical fish collection locations can greatly aid in identifying potential fish community changes in systems where monitoring is limited.

Environmental DNA metabarcoding reveals the effect of river slope on diadromous fish communities in island rivers

Understanding the natural factors that influence diadromous fish communities in river ecosystems is crucial for effective biodiversity management. However, this remains an unexplored and challenging task. Here, the relationship between river morphology and diadromous fish communities within rivers was explored. We focused on river slope and estuary size, as these may constrain their upstream migration and affect the availability of important nursery grounds for some species. Using environmental DNA metabarcoding, we investigated the diadromous fish communities within 12 rivers of subtropical Yakushima Island, southern Japan, and compared the community differences among rivers with different river slopes and estuary sizes. Fewer diadromous fish species were detected in rivers with steeper slopes and smaller estuaries. Additionally, the community similarity was greater among rivers with analogous gradients and estuary sizes. These results indicate that river slopes exert a species-selective effect on diadromous fish communities by affecting their upstream migration and estuarine habitat availability, thereby playing a crucial role in shaping the composition of these communities within rivers. Conserving gentle-gradient rivers with large estuaries would be crucial for preserving the diversity of diadromous fish species, which play a crucial role in shaping biodiversity especially in rivers of subtropical and tropical oceanic islands.

MANAGEMENT PLAN FOR RECREATIONAL FISHING IN THE HYDROGRAPHIC ÂNCORA RIVER BASIN

This work is the result of the most recent studies carried out by the ICNF in the drainage basin of the future recreational fishing area (ZPL) of the Âncora river. These supported the design of a plan for the sustainable management and exploitation of recreational fishing in this river basin, which is part of the Minho and Lima Hydrographic Region (RH1). Seven sampling stations were set up for this purpose. Six in the main watercourse and one in the Regueiro dos Enxurros, in the middle of an area of successive waterfalls, just before its confluence with the Regueiro da Lapa Ladrão, which is considered to be the source of the Âncora River. At these stations, hydromorphological quality assessments were carried out on 500 m stretches using the River Habitat Survey (RHS) methodology and fish community monitoring (three per station, for a total of twenty-one inventories) using the electrofishing technique. The results made it possible to consider implementing more sustainable management and exploitation measures for fishing in these continental bodies of water. In this context, the management options took marked account of conservation objectives associated with the presence of European eels (Anguilla anguilla) and red eels (Achondrostoma spp.), the wide distribution of trout (Salmo trutta) and the reduced presence of boga (Pseudochondrostoma duriense) whose distribution in the upper third of this drainage basin is conditioned by a very peculiar geomorphology, filled with numerous slopes, which give rise to beautiful waterfalls. With the information collected, data on the age, growth and physical condition of the ichthyofauna assessed was studied, and their relationship with the different types of habitat in which they occurred was established. The results show that these are watercourses with "Excellent" quality habitats, but with levels of artificialization ranging from "severely modified" to "predominantly unmodified", the result of anthropogenic actions over time. Trout (Salmo trutta) are the most widely distributed species along the main watercourse. The absence of exotic aquaculture species in the samples and the lack of medium to large water impoundment structures in the hydrographic network evaluated should be emphasized. The existing river compartmentalization is materialized by the numerous micro-weirs (many of which allow aquatic organisms to migrate for a good part of the year) and a few waterfalls (natural cascades) that prevent fish from passing through. The trout show isometric growth for all the sampling stations, a balanced physical robustness and a maximum lifespan of 6 years. As a practical result of this study, a Recreational Fishing Management and Exploration Plan for the Âncora River will be created, which will support the design of measures to promote the sustainable management of recreational fishing in the area of the future LPZ, reconciling this activity with nature conservation objectives.

Temporal change in urban fish biodiversity-Gains, losses, and drivers of change.

Our aim was to examine temporal change in alpha and beta diversity of freshwater fish communities in rivers that have urbanized over the same period to understand the influence of changes in land use and river connectivity on community change. We used biological (2001-2018), land use (2000-2015), and connectivity data (1987-2017) from Toronto, Ontario, Canada. We used linear mixed effects models to determine the strength of upstream land use, connectivity, and their changes over time to explain temporal change in alpha and beta diversity indices. We examined beta diversity using the temporal beta diversity index (TBI) to assess site-specific community change.The TBI was partitioned into gains and losses, and species-specific changes in abundance were assessed using paired t-tests. There were more gains than losses across the study sites as measured by TBI. We found little to no significant differences in species-specific abundances at aggregated spatial scales (study region, watershed, stream order). We found different relationships between landscape and connectivity variables with the biodiversity indices tested; however, almost all estimated confidence intervals overlapped with zero and had low goodness-of-fit. More fish biodiversity gains than losses were found across the study region, as measured by TBI. We found TBI to be a useful indicator of change as it identifies key sites to further investigate. We found two high value TBI sites gained non-native species, and one site shifted from a cool-water to warm-water species dominated community, both of which have management implications. Upstream catchment land use and connectivity had poor explanatory power for change in the measured biodiversity indices. Ultimately, such spatial-temporal datasets are invaluable and can reveal trends in biodiversity useful for environmental management when considering competing interests involved with urban sprawl in the ongoing "Decade on Restoration."

Assessing electrofishing effort required to estimate species richness of fishes in nonwadeable rivers of Alabama, USA

AbstractObjectiveMuch of the fish biodiversity in the southeastern USA is found in lotic systems, but assessment of fish community structure in large nonwadeable rivers has lagged compared to streams. Sampling designs that adequately estimate species richness are a prerequisite to creating bioassessment metrics but thus far has not been attempted in highly biodiverse rivers in the southeastern USA east of the Mississippi River.MethodsFish communities in four nonwadeable rivers across Alabama spanning catchments of 2046–57,415 km2 were sampled using electrofishing in summer and fall of 2015 and 2016. All rivers were sampled during day; two rivers were also sampled at night to assess diel changes in species accumulation.ResultIn total, 55,239 fishes were collected across rivers, representing 106 species in 24 families. Species collected continued to increase as sampling increased in all river‐season combinations, rarely displaying an asymptote. Species generally accrued more quickly and accumulation curves showed more tendency to asymptote in fall than summer in all but the largest river. Species accumulated faster at night than during the day in both rivers during summer but was similar between diel periods in fall. On average, 95% of the species observed were collected in eight subreaches in fall but not until nine during summer; however, observed number of species was generally one to six species lower than mean estimates of true species richness. Sampling efforts collected approximately similar species assemblages across transects, years, season, and diel periods. Extrapolation of species accumulation curves suggested that 6–17 km of sampling distance would be required to collect all species present.ConclusionResults of this study demonstrate that assessing fish communities in species nonwadeable rivers requires an inordinate amount of sampling effort, which may be unattainable for many agencies. Further study is warranted to develop innovative solutions to overcome these logistical challenges.

LITERATUR REVIEW: PEMANFAATAN METABARCODING DNA LINGKUNGAN UNTUK MENGANALISIS KEANEKARAGAMAN HAYATI IKAN

Background: Environmental DNA (eDNA) metabarcoding is a new method for assessing biodiversity in which samples are taken from the environment via water, sediment or air from which DNA is extracted, and then amplified using common or universal primers in a polymerase chain reaction and sequenced using next-generation DNA. sequencing to produce thousands to millions of sequences. From this data, the presence of species can be determined, and overall biodiversity can be assessed.. Methods: The method used is a literature review using reputable national and international journals Results: Based on the results of the literature review, the use of eDNA metabarcoding can be used to analyze fish biodiversity. eDNA metabarcoding can be analyzed with primary data, databases, and bioinformatic pipelines. Conclusion: eDNA metabarcoding has been widely used to study fish communities in lakes and rivers, as well as coastal fish communities. Despite its considerable ability to detect fish species in large aquatic systems, many methods still need to be understood to improve the quality of primers, databases, and pipelines for analyzing fish biodiversity

Invasive poeciliids dominate fish community in a highly altered river: insights from a diversity study of riverbank fishes in Mexico

Abstract The presence of invasive species can cause significant changes in native communities and ecosystem functions. Mexico is home to 6% of all known freshwater fish species on the planet, with a high rate of endemism. Due to heavy urbanization, the Mexican Central Plateau has become one of the most densely populated areas in the world, and its Tula River is considered one of the most polluted rivers in Mexico. Our objective was to investigate whether native fish species persist in such adverse conditions and to evaluate the seasonal and spatial distribution of both native and non-native species at three sites along the Tula River. We evaluated environmental characteristics and fish community structure. We found two native species, the black fin goodea (Goodea atripinnis) and the yellow shiner (Notropis calientis). However, their abundance was extremely low across all sites and seasons. In contrast, invasive poecilids dominated the communities, accounting for 99.4% of the total abundance. Our results indicate a clear relationship between river characteristics and fish community structure, highlighting the significance of river width, river velocity, temperature, dissolved oxygen, and pH. The prevalence of invasive species underscores the urgent need for conservation efforts aimed to protect and restore native fish populations.

Environmental DNA biomonitoring reveals the human impacts on native and non-native fish communities in subtropical river systems

Subtropical rivers are one of the hotspots of global biodiversity, facing increased risks of fish diversity changes and species extinction. However, until now, human impacts on native and non-native fish communities in subtropical rivers still lack sufficient effort. Here, we used the environmental DNA (eDNA) approach to investigate fish communities in the Dongjiang River of southeast China, a typical subtropical river, and explored the effects of regional land use and local water pollution on fish taxonomic and functional diversity. Our data showed that 90 species or genera of native fish and 15 species or genera of non-native fish were detected by the eDNA approach, and there was over 85% overlap between eDNA datasets and historical records. The taxonomic and functional diversity of all, native and non-native fish communities showed consistent spatial patterns, that is, the upstream of the tributary was significantly higher than that of the mainstream and downstream. Land use and water pollution such as COD and TP were the determinants in shaping the spatial structure of fish communities, and water pollution explained 31.56%, 29.88%, and 27.80% of the structural variation in all, native and non-native fish communities, respectively. The Shannon diversity and functional richness of native fish showed a significant downward trend driven by COD (pShannon = 0.0374; pfunctional = 0.0215) and land use (pShannon = 0.0159; pfunctional = 0.0441), but they did not have significant impacts on non-native fish communities. Overall, this study emphasizes the inconsistent response of native and non-native fish communities to human impacts in subtropical rivers, and managers need to develop strategies tailored to specific fish species to effectively protect water security and rivers.

Deforestation strengthens environmental filtering and competitive exclusion in Neotropical streams and rivers.

Understanding how anthropization impacts the assembly of species onto communities is pivotal to go beyond the observation of biodiversity changes and reveal how disturbances affect the environmental and biotic processes shaping biodiversity. Here, we propose a simple framework to measure the assembly processes underpinning functional convergence/divergence patterns. We applied this framework to northern Amazonian fish communities inventoried using environmental DNA in 35 stream sites and 64 river sites. We found that the harsh and unstable environmental conditions characterizing streams conveyed communities towards functional convergence, by filtering traits related to food acquisition and, to a lower extent, dispersal. Such environmental filtering also strengthened competition by excluding species having less competitive food acquisition traits. Instead, random species assembly was more marked in river communities, which may be explained by the downstream position of rivers facilitating the dispersion of species. Although fish assembly rules differed between streams and river fish communities, anthropogenic disturbances reduced functional divergence in both ecosystems, with a reinforcement of both environmental filtering and weaker competitor exclusion. This may explain the substantial biodiversity alterations observed under slight deforestation levels in Neotropical freshwater ecosystems and underlines their vulnerability to anthropic disturbances that not only affect species persistence but also modify community assembly rules.

Morphological and Trophic Features of the Invasive Babka gymnotrachelus (Gobiidae) in the Plain and Mountainous Ecosystems of the Dniester Basin: Spatiotemporal Expansion and Possible Threats to Native Fishes

Over the last few years, the racer goby B. gymnotrachelus quickly expanded in the mountainous rivers of the Dniester basin at aheight of 300 m a.s.l. and above. The occurrence frequency of the racer goby in fish communities in the mountainous rivers of the Carpathian ecoregion remains low (up to 20%), as compared with the plain rivers, where the species occurs in 70–100% of cases. The major prey groups in its diet in both plain and mountainous were Chironomidae, Diptera, and Crustacea. Chironomidae formed the maximal portion (35% at an occurrence frequency of 72.5%), and Trichoptera formed 18%at an occurrence frequency of 41%, whereas fish larvae were absent in their diet in mountainous rivers. The peculiar environmental conditions of mountainous rivers caused the adaptive modifications of the morphological features of the racer goby at the subpopulation level, which compriseda decrease inthe specimens’ size in rivers with a flow velocity of above 1.5 m/s and rivers with pebble contentsof above 50%in the bottom sediments.The modifications showed an increase inbody streamlining as an adaptation to flow velocity and turbulence and the stony substrate of the river’s bottom. Thus, the morphological adaptation of B. gymnotrachelus to the conditions of the mountainous rivers is in progress, and the formationof astable population in these rivers can be expected. In contrast, the native fish species’ resilience in the face of newcomers is still high, and this is because their alevins are not food for invaders.

Unveiling the rise of non-native fishes in eastern alpine mountain rivers: Population trends and implications.

Insufficient knowledge about the occurrence and spread of non-native fish in mountain regions has impeded effective management strategies worldwide. To address this gap, this study analysed over 1300 electrofishing surveys across 650 sites, encompassing a vast 7400 km2 area in the Eastern Alps. The primary objectives were to quantify the occurrence of non-native species and predict their spread in different river types. Furthermore, the study estimated population sizes and biomass trends for over 150 sites that were surveyed multiple times between 2000 and 2020. Out of the 42 fish species in the study region, 11 were identified as non-native. Notably, two invasive species of Union concern, Lepomis gibbosus and Pseudorasbora parva, increased their population sizes by 8% and 9% per year, over the past decades, supposedly supported by increasing water temperatures. Among the non-native species relevant for recreational fishing, Oncorhynchus mykiss populations showed a significant increase of approximately 7% per year, Salmo trutta populations remained stable, and Salvelinus fontinalis populations experienced a notable decline of approximately 7.4% per year. These varying population trends may be attributed to disparities in stocking intensities, with S. fontinalis receiving minimal stocking compared to the other species. This study revealed that non-native and invasive fish species are a relevant part of fish communities in mountain rivers. Non-salmonid non-natives thrive in warm rivers at lower elevations, whereas salmonid non-natives consolidate in steeper habitats. Because rising temperatures in mountain rivers will accelerate the spread and growth of these species, this first quantification of the current extent will improve fish management strategies in mountainous areas.

Using eDNA Metabarcoding for the Analysis of Fish Communities in Rivers

The present study aims to assess the effectiveness of eDNA metabarcoding for the analysis of fish communities in rivers. The findings suggest that the methodology attained an accuracy rate of 80% for species identification and was able to detect between 79.2% and 87.5% of the fish species recorded in the fish monitoring database for three rivers of Anseong, Bokha and Gyeongan during the period 2019 to 2021. Significantly, the eDNA metabarcoding technique enabled the successful detection of comparatively larger fish species, such as <i>Channa argus</i> and <i>Silurus asotus</i>. Furthermore, depending on the river, this method identified 12 to 14 additional species that could not be observed through traditional methodologies. However, it is worth noting that the Mifish primer used amplifies short gene segments, which can pose challenges in identifying species with identical gene sequences. Notwithstanding this limitation, the advantages of eDNA analysis over conventional methods are significant, enabling the identification of a broader range of species within a shorter timeframe, using smaller sample volumes and minimizing risks to both endangered fish and to researchers. As a result, eDNA analysis represents a valuable alternative for assessing biodiversity and for collecting data on fish species that are challenging to analyze.

The Ohio River Ecological Research Program (ORERP): History and Accomplishments

The Ohio River Ecological Research Program (ORERP), sponsored by multiple power plants along the river's main stem, was in existence for 50 years. Its goals were to collect fish community data to evaluate possible impacts to Ohio River fish communities because of either power plant discharges (i.e., thermal effects) or cooling water intakes. Studies by the ORERP have shown that thermal avoidance of discharge areas often occurs during the summer, but this avoidance is temporary and reversible, and long‐term disruptions of the fish communities have not been detected. Impingement studies conducted at 13 plants from 2005 to 2007 showed that impingement was highly episodic and dominated by clupeids and Freshwater Drum Aplodinotus grunniens. Impinged fish were mainly (>90%) young of the year; as a result, impingement numbers strongly reflected year‐class strength. Entrainment of ichthyoplankton was conducted under the ORERP at seven plants in 2015 and 2016. Annual entrainment estimates ranged from 43 million to 3.5 billion (i.e., 3.5 × 109) ichthyoplankton depending on the plant. The principal benefit of this collaborative effort was that these data allowed the sponsoring facilities and others to access an extensive database upon which fact‐based regulatory, resource management, and plant operating decisions could be based. Most importantly, the ORERP is a collaborative model for private and public organizations to collect, share, and analyze data and make resource management decisions for a large, multi‐use aquatic resource.

Using environmental DNA metabarcoding to monitor fish communities in small rivers and large brooks: Insights on the spatial scale of information

Monitoring fish communities is central to the evaluation of ecological health of rivers. Both presence/absence of fish species and their relative quantity in local fish assemblages are crucial parameters to measure. Fish communities in lotic systems are traditionally monitored via electrofishing, characterized by a known limited efficiency and high survey costs. Analysis of environmental DNA could serve as a non-destructive alternative for detection and quantification of lotic fish communities, but this approach still requires further insights in practical sampling schemes incorporating transport and dilution of the eDNA particles; optimization of predictive power and quality assurance of the molecular detection method. Via a controlled cage experiment, we aim to extend the knowledge on streamreach of eDNA in small rivers and large brooks, as laid out in the European Water Framework Directive's water typology. Using a high and low source biomass in two river transects of a species-poor river characterized by contrasting river discharge rates, we found strong and significant correlations between the eDNA relative species abundances and the relative biomass per species in the cage community. Despite a decreasing correlation over distance, the underlying community composition remained stable from 25 to 300 m, or up to 1 km downstream of the eDNA source, depending on the river discharge rate. Such decrease in similarity between relative source biomass and the corresponding eDNA-based community profile with increasing distance downstream from the source, might be attributed to variation in species-specific eDNA persistence. Our findings offer crucial insights on eDNA behaviour and characterization of riverine fish communities. We conclude that water sampled from a relatively small river offers an adequate eDNA snapshot of the total fish community in the 300–1000 m upstream transect. The potential application for other river systems is further discussed.

Understanding the temporal dynamics of a lowland river fish community at a hazardous intake and floodgate to inform safe operation

Entrainment and mortality of freshwater fish at hazardous pumping station intakes used for Flood Risk Management (FRM) are of global concern. Although upstream and downstream passage of diadromous fish has received considerable attention, the ecological behaviours of river-resident fish at these structures and how to protect these species from entrainment is poorly-understood. At a lowland flood-relief pumping station and floodgate situated off-channel (River Foss) to the main-river Yorkshire Ouse (York, England), multi-beam sonar (Dual-Frequency Identification Sonar: DIDSON) was used over a pluriannual (three years) period to investigate diel movements of river-resident fish in response to the variations in temperature, hydrology and pump and floodgate operation, and to determine fish-friendly management options. Diel lateral movements of thousands of river-resident fish between the main-river, floodgate operated channel (River Foss) and off-channel pump forebay were predominantly during the crepuscular period and daytime, proposing important considerations for when managers should operate pumps and associated flood infrastructure. Seasonal diel movements increased throughout winter during a baseline year (no pump operation) and overwintering behaviour was influenced by cooling river temperatures. A Generalized Linear Mixed Model (GLMM) revealed fish entered the off-channel forebay when river levels were stable and not when they were rising or falling, suggesting hydrological stability was important for the ecological function of this fish community. Two years of impact data (pumps operated) then revealed pump operations severely disrupted the ecological functions of local fish populations, which was also uniquely quantified over two independent 24h periods during which temporal fish counts were reduced by 85%. A trial period where the floodgate was lowered ahead of dawn significantly reduced fish immigration into the hazardous forebay when compared to two different hydrological periods. Modifying when the floodgate and pumps operate, including lowering the floodgate ahead of fish immigration at dawn, and starting pumps during the night (but not day), are therefore promising non-engineered management options to prevent immigration of fish into the hazardous off-channel pump forebay and to reduce entrainment and mortality risk during pump operation.

Anthropogenic nutrient loading affects both individual species and the trophic structure of river fish communities

Although the concept of trophic interactions has been used for a long time, there are still considerable gaps in our understanding of the effect of various environmental factors on trophic interactions within river fish assemblages. Carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of 20 species of fish belonging to both eurytopic and rheophilic ecological groups from a large temperate rivers were used to evaluate overall trophic niche use and trophic position of species, and to find out how environmental variability associated with nutrient loading affects individual and community-wide aspects of trophic structure. The study was carried out at 11 sites along the European rivers Vltava and Elbe, representing a continuous gradient of pollution and habitat degradation. Corrected Standard Ellipse Area (SEAc) was significantly larger for the group of eurytopic ecological species than for rheophilic species. Despite narrower isotopic niche space, rheophilic fish species occupied a higher trophic position, suggesting that these species use resources more enriched in 15N. Of the 11 environmental variables tested, nutrients had a significant effect on trophic niche area of species (SEAc), indicating that eutrophication is of critical importance for fish assemblages. Isotopic niche area of species was found to be positively influenced by total phosphorus, and negatively affected by concentrations of nitrate (N-NO3−) and ammonia (N-NH4+). A negative association between oxygen demand and a measure of trophic diversity - mean distance to centroid (CD)- and a measure of density and clustering of species - mean nearest neighbor distance (MNND)- were found, indicating that the oxygen demand is a key factor influencing community trophic structure. An observed pattern where nutrient loading influenced both individual species and trophic structure of the fish assemblage provides strong support for an anthropogenic influence on riverine food webs.

The Ichthyofauna of the Bednja River, Ichthyological ‘Hot Spot’ in the Danube Basin—Exceptional Diversity under Strong Threats

The Bednja River is the longest river flowing with its full course exclusively through Croatia and one of the largest right tributaries of the Drava River, which belongs to the Danube River basin. Due to the variety of habitats present within the course of the Bednja River, as well as older literature reports, it can be expected that this river harbors a high number of freshwater fish species and would likely benefit from conservation and preservation efforts. We compiled and analyzed the existing literature data on the Bednja River fish communities and performed field investigations at various localities on this river in order to describe the current structure of its freshwater fish fauna and monitor changes induced by human activities. Our results corroborate the presence of a rich and diverse fish community. Moreover, with 36 native species, the Bednja River harbors one of the richest fish communities in Croatia as well as in the Danube basin. Unfortunately, modifications of the native fish community were evident in the form of local extinctions, reductions in the population abundance of several native species and the presence of non-native species. Habitat degradation and fragmentation were identified as the most serious threats provoking negative effects on the native fish populations, followed by predatory and competitive effects of invasive species.

Alien species and climate change drive shifts in a riverine fish community and trait compositions over 35 years

Alien fish substantially impact aquatic communities. However, their effects on trait composition remain poorly understood, especially at large spatiotemporal scales. Here, we used long-term biomonitoring data (1984–2018) from 31 fish communities of the Rhine river in Germany to investigate compositional and functional changes over time. Average total community richness increased by 49 %: it was stable until 2004, then declined until 2010, before increasing until 2018. Average abundance decreased by 9 %. Starting from 198 individuals/m2 in 1984 abundance largely declined to 23 individuals/m2 in 2010 (−88 %), and then consequently increased by 678 % up to 180 individuals/m2 until 2018. Increases in abundance and richness starting around 2010 were mainly driven by the establishment of alien species: while alien species represented 5 % of all species and 0.1 % of total individuals in 1993, it increased to 30 % (7 species) and 32 % of individuals in 2018. Concomitant to the increase in alien species, average native species richness and abundance declined by 26 % and 50 % respectively. We identified increases in temperature, precipitation, abundance and richness of alien fish driving compositional changes after 2010. To get more insights on the impacts of alien species on fish communities, we used 12 biological and 13 ecological traits to compute four trait metrics each. Ecological trait dispersion increased before 2010, probably due to diminishing ecologically similar native species. No changes in trait metrics were measured after 2010, albeit relative shares of expressed trait modalities significantly changing. The observed shift in trait modalities suggested the introduction of new species carrying similar and novel trait modalities. Our results revealed significant changes in taxonomic and trait compositions following alien fish introductions and climatic change. To conclude, our analyses show taxonomic and functional changes in the Rhine river over 35 years, likely indicative of future changes in ecosystem services.

Ecological integrity of a medium‐sized river of a lake basin towards ecosystem health management

AbstractThis study demonstrates the use of a multi‐metric Fish Index of Biotic Integrity (FIBI) for assessment of ecological health of a medium‐sized river, under a gradient of human disturbances in the Kenyan side of the Lake Victoria Basin (LVB). Development of the index is based on water quality, river habitat quality and fish community attributes. The river was sampled from 2019 to 2020 during wet and dry seasons of the year at nine selected sites along its length based on the level of human disturbance. A total of nine metrics representing the structural and functional organization of fish communities were qualified using the separation power of Mann–Whitney U test (p < .05). Metric values at unimpaired, intermediate, impaired and no‐fish sites were used to establish the scoring criteria using inter‐quartile ranges. A 1, 3, 5 scoring system representing high, fair and slight deviation from the best site, respectively, was used in the final FIBI development. River mouth station had the highest FIBI score of 43, with extensive natural riparian buffer vegetation (>20 m) and 18 fish species implying least human disturbance. The midstream section had intermediate FIBI scores of 34–36, with wide natural riparian buffer vegetation (10–20 m) and 6–9 fish species that represented moderate human disturbance. The upstream section had the least FIBI score of 0, with no fish and narrow riparian buffer vegetation (<5 m) that indicated notable degradation. This study presents the preliminary results of using fish and habitat metrics for development of an Index of Biotic Integrity (IBI) for a medium‐sized river in a lake basin, as a decision‐making support tool for effective management and conservation of water resources. The study recommends restoration of river habitat quality for improved ecological integrity of rivers in the Lake Victoria Basin.