Natural and artificial thermal water bodies in temperate regions represent hotspots of non-native species, including those of pet-traded origins. These systems provide valuable in situ settings for investigating hard-to-come-by information such as seasonal population dynamics, adaptive potential of (sub)tropical non-native species, and interactions between native and non-native species. Here, we investigated seasonal population dynamics of Poecilia reticulata and co-occurring native and non-native taxa along a thermal gradient within the naturally heated waters of the Fontcaude thermal park, Southern France. Based on temperature data and the presence of physical barriers, we divided the water into three thermal zones: the pond, the upper channel and the lower channel. With thermal zones playing a minor role, the occurrence and abundance of native species were mainly driven by seasonality. In contrast, non-native species were less affected by seasonality but were strongly positively associated with the warmer areas represented by the pond and the upper channel. While non-native species were omnipresent, community composition varied both spatially and temporally. Such changes were often species-specific and reflected differences in thermal tolerance, ontogeny and sex. We further explored P. reticulata’s potential introduction pathways, using DNA barcoding to compare feral and worldwide pet-traded individuals and provide an updated distribution of established feral populations across Europe, revealing six previously unreported established populations. These findings highlight the importance of thermal refuges as permanent establishment sites for non-native species in otherwise climatically unsuitable temperate regions.
Hydropower provides renewable, fossil-free energy, but negatively impacts riverine ecosystems. Innovative assessment methods are needed to find mitigation measures protecting biodiversity, especially in the context of climate change, that balance hydropower production and ecological restoration. We developed such methods for the heavily regulated Ume River catchment in northern Sweden, and identified environmental measures that could be implemented without considerable negative effects on hydropower production. A successful collaboration process involving local communities, authorities, hydropower operators, and researchers resulted in 10 categories of environmental mitigation measures. Implementation of the suggested measures in the main channel would result in 306 ha of new lotic habitat and improved quality of another 240 ha. Implementation of measures in tributaries would add 36 ha of lotic habitat for the biota in the main channel and give access to an additional 360 km of unregulated river habitat. Our study shows the importance of unregulated tributaries in maintaining habitat diversity and connectivity pathways of benefit to migratory riverine biota in the catchment. Heavily regulated rivers should not be exempt from assessment of the feasibility of the ecosystem mitigation measures, since there may be rehabilitation measures that can be implemented with little negative impact on hydropower production.
Fish baiting is an important pathway for the introduction of biogenic elements, particularly nitrogen and phosphorus, into freshwater ecosystems, and groundbaits may also contain heavy metals that intensify environmental degradation. This study assessed the extent of groundbait use in Poland and its potential ecological impacts, addressing a globally under-recognised source of anthropogenic pressure on inland waters. This study combined a survey of 1,574 anglers with chemical analyses of 12 commercially available groundbait mixtures, quantifying the nutrient content and concentrations of Hg, Cd, Pb, and As. Aquarium experiments were conducted to examine the nutrient release dynamics and associated changes in selected physicochemical water parameters. More than 60% of the respondents reported the regular use of groundbaits, predominantly ready-made commercial products. Chemical analyses revealed pronounced variability among the mixtures, with carp-oriented groundbaits exhibiting the highest concentrations of nutrients and heavy metals. All analysed baits contained detectable levels of heavy metals, raising concerns regarding food safety and human health. Experimental results demonstrated rapid nutrient release, leading to measurable changes in dissolved oxygen, pH and conductivity. Overall, groundbait use was identified as a significant yet often overlooked source of eutrophication and toxicological pressure on freshwater ecosystems, highlighting the need for responsible use guidelines and further field-based research.
Freshwater mussels of the family Unionidae require an obligatory parasitic larval stage on a specific fish host species to complete their ontogeny. This relationship can be highly specific to co-evolving species and geographically variable, although information regarding host suitability remains scarce. Recent advances in molecular and phylogenetic studies of fish have uncovered cryptic host species that have yet to be tested for their role in mussel development. The genus Cottus, particularly European bullhead Cottus gobio, is known to be a primary host for the threatened thick-shelled river mussel Unio crassus complex, a group of cryptic species of large freshwater mussels endemic to Europe. However, the recent revision of the geographical distribution of another Cottus species, Baltic sculpin Cottus microstomus, raises questions about its suitability as a host for species in U. crassus complex. We report that the transformation of mussel larvae from mixed population of U. crassus sensu stricto and U. nanus into juvenile mussels is successful when C. microstomus serves as the host, with an effectiveness comparable to that of the well-established host Eurasian minnow Phoxinus phoxinus. The sympatric distribution of C. microstomus and U. crassus complex is confined to several river drainages in Central and Eastern Europe, areas that are affected by anthropogenic factors, which may pose challenges to their conservation.
Eutrophication caused by excess inputs of nitrogen (N) and phosphorus (P) is a global environmental problem for lakes. External nutrient control may change the relative influence of N and P on phytoplankton biomass. For Lake Wuli, a subtropical shallow lake in China, suffering from severe eutrophication since the early 1980s, a restoration program aimed to control external nutrient loading was implemented in 2002. Long-term data (1998–2019) revealed that annual mean phytoplankton biomass (Chl a concentration), total nitrogen (TN), and total phosphorus (TP) concentrations decreased significantly following external nutrient loading reduction. Both summer and winter TN concentrations declined markedly, whereas TP and Chl a decreased significantly only in winter. The decline in the concentrations of TN was larger than that of TP, resulting in a significant reduction in the TN/TP ratio from 33.3 to 15.5 on an annual basis, from 23.3 to 10.9 in summer, and from 44.3 to 21.2 in winter. A high TN/TP ratio and low SRP concentration (<10 μg/L) indicate that Lake Wuli was P-limited in winter. The changes in the summer TN/TP ratio indicate a shift from P limitation of phytoplankton growth to N and P co-limitation, which was confirmed by a nutrient addition experiment conducted in summer in 2023. Our results indicate that, at the present environmental conditions, inputs of either N or P, and especially both nutrients, will increase the risk of phytoplankton blooming in summer in this subtropical shallow lake.
Environmental DNA (eDNA) is widely applied to monitor aquatic biodiversity, but false-positive detections may arise when DNA from consumed or processed organisms is discharged to waterways through domestic wastewater. We experimentally tested whether such wastewater-derived eDNA persists through municipal treatment and contributes to downstream false-positive detections. Controlled spikes of homogenized tissue from six target taxa (two marine fishes and four freshwater fish/amphibians) were introduced into the influent of four wastewater treatment plants (WWTPs) in Flanders, Belgium. Water samples were collected at three points: influent, treated effluent, and the receiving river. Sampling was performed before and up to 24 h after spiking, and eDNA concentrations of each of the six target taxa were quantified using droplet digital PCR. All species showed post-spike increases in influent eDNA concentrations, although individual taxa were not detected in every influent sample, particularly at the largest facilities where hydraulic dilution reduced short-term detectability. In contrast, effluent and downstream concentrations remained zero across all WWTPs, indicating that no spiked eDNA persisted beyond the treatment process. One WWTP located near a marine harbor showed background detection of marine fish species even before spiking, likely originating from seafood processing or domestic discharge. However, these signals were also fully removed during treatment. Overall, modern secondary treatment effectively eliminates exogenous eDNA, preventing its release into receiving waters. While systems with incomplete sewer connectivity or combined sewer overflows may still discharge untreated domestic wastewater, under standard operating conditions riverine eDNA detections can be interpreted as reflecting local biota rather than wastewater contamination.
Branchiobdellids are small ectosymbiotic annelids living primarily on freshwater crayfish. Because humans frequently translocate crayfish, these symbionts can be co-translocated, with implications for biosecurity and conservation. Despite over a century of study, relationships between European branchiobdellids and their crayfish hosts (basibionts) remain poorly resolved. Therefore, we analysed 215 mitochondrial COI sequences (67 new and 148 from GenBank and BOLD databases) spanning most documented ectosymbiont–basibiont associations. Phylogenetic reconstruction and species delimitation identified seven recognized species and revealed deep intraspecific divergence in five, suggesting the presence of cryptic lineages. Branchiobdella hexadonta and Branchiobdella parasita each contained multiple genetically distinct groups linked to different crayfish species, whereas Branchiobdella astaci and Branchiobdella balcanica showed strong host-specific monophyly. Branchiobdella italica and Branchiobdella pentadonta showed lower divergence, consistent with a younger origin inferred by molecular clock analyses. Divergence times broadly matched crayfish basibiont history, indicating likely co-speciation. Results support an origin of Branchiobdella inhabiting crayfish gill chambers and show that crayfish translocations may blur natural epibiont–basibiont boundaries, thus complicating monitoring of symbiont lineages. This work advances our understanding of co-evolution in freshwater ecosystems and underscores the conservation importance of preserving both native crayfish and their associated symbionts, whose extinction risks may be underestimated due to unresolved taxonomy.
Climate change is causing changes in weather patterns and has led to prolonged droughts. Much of western North America has experienced recent droughts, especially in California, which experienced a historic drought from 2012–2016. Drought can significantly impact aquatic turtles, especially those populations existing in ephemeral habitats. Much of the range of western pond turtles (Actinemys) has been subject to recent droughts. In 2017, after the worst of the drought had passed, I sampled a large population of Actinemys along the southern flank of the Tehachapi Mountains, California, which I first trapped in 2007 and 2010. I trapped the small pond in four of seven years between 2017 and 2023 when there was much less water than before. I caught only 15% (48/321) of the number of turtles I caught before the drought, but there were adult turtles that survived from earlier trapping, unmarked adults that were alive in 2010, young turtles, and many reproductive females. Growth rates were lower than previously, but growth rates were increasing in the last 2 years of trapping. Survivorship in 2017–2023 was similar to that in 2007/2010. Unlike many small ponds in the range of Actinemys that are ephemeral, relying on water input from the surrounding watershed, the study pond is spring-fed. Intermittent years of high rainfall (>500 mm) likely have recharged the surrounding aquifer that keep this pond with some water even in drought years, allowing the population of Actinemys at Gorman Pond to survive an extended drought period.
To address increasing demand for ecological models of aquatic species that can inform the management of national freshwater resources, we leveraged manager input to develop suites of environmental data layers characterizing freshwater habitats for the contiguous United States. Using the National Hydrography Dataset, these new data cover lentic and lotic systems under current and near-future environmental conditions. The data include a variety of covariate categories including climate, soil chemistry, land use and land cover, and human modification of the surrounding landscape. The predictor resolution for atmospheric climate predictors was the lake (wetland) or stream reach, and, for the terrestrial proxies, the subwatershed (HUC12) surrounding the lake or stream reach was chosen to capture the relevant land features surrounding the habitat. Future land use, land cover and streamflow predictions were included from present to mid-century. These data are available for the development of freshwater ecological models in the contiguous United States for a variety of applications, including species distribution modeling and exploring change in spatially diverse aquatic systems in time.
Environmental DNA (eDNA) is increasingly used to estimate species abundance and biomass, but impacts of sampling strategies are poorly understood. We compared point and spatially integrated sampling, two commonly used sampling strategies, in two lakes using quantitative eDNA analysis. Integrated sampling increases the spatial coverage and the total volume of water sampled relative to point sampling. However, integrated sampling led to lower fish eDNA recovery and underestimated eDNA concentrations compared to point sampling. Although the exact mechanisms underlying this pattern require further investigation, these results underscore the importance of aligning sampling strategies with key study objectives.
Invasive alien crayfish pose significant threats to biodiversity in aquatic ecosystems. In isolated water bodies they impact local amphibian populations by predation, and altering water quality due to burrowing behaviour. Eight pools in either natural and urban areas were selected, where either signal crayfish, red swamp crayfish, or both species occurred. During six years, we assessed three methods to reduce crayfish abundances: biannual crayfish trapping and removal, trapping combined with drainage of pools, and trapping combined with introducing eels as predators. Trapping alone was insufficient to stagnate population growth of red swamp crayfish, though it slowed. Trapping + draining proved ineffective, since red swamp crayfish hid in burrows and aquatic vegetation. The trapping + eels method, reduced population growth and mean length of the red swamp crayfish. Furthermore, we observed a strong population decline in signal crayfish when the red swamp crayfish colonised a pool. Our findings demonstrate that crayfish negatively impact populations of several amphibian species. In natural pools, amphibian numbers increased when crayfish numbers decreased. Urban pools, which showed an increase in crayfish numbers, displayed a decline in amphibians. A combination of predator introduction and removal of crayfish could provide a sustainable solution for controlling crayfish populations and improving amphibian habitats.
Proliferative kidney disease (PKD) poses a threat to wild salmonids, yet its spatial patterns remain poorly understood, particularly at large scales and across varied ecosystems. We combined the largest nationwide screening of brown trout (Salmo trutta), consisting of 1072 fish from 155 locations, spanning a 1480 km latitudinal gradient, with process-based stream-temperature modelling. From this data, we map infection by the parasite Tetracapsuloides bryosalmonae (T.b), quantify parasite load (using qPCR from kidney tissue), and calculate renal hyperplasia across Sweden under different thermal regimes. PKD emerged if study-period mean water temperatures approached a threshold of approximately 15.4 °C and renal hyperplasia peaked near 17 °C; however, warm water did not always cause disease: asymptomatic individuals were common above the temperature threshold. Spatial mixed-effects models revealed that parasite load and temperature interacted to determine disease severity, whereby severe PKD was associated with lower parasite loads under warmer conditions. Furthermore, deviations from broad latitudinal patterns were observed, where distinct coastal hotspots in central Sweden would be overlooked by assuming a simple temperature-driven latitudinal gradient. Our model shows that such patterns remain after accounting for temperature and parasite load, indicating that local conditions and additional environmental drivers are likely affecting epidemiology at relatively small spatial scales. Management actions that moderate stream temperatures, such as riparian shading or the removal of impoundments, or reduce suitable habitat for bryozoan hosts, may therefore mitigate disease impacts under a warming climate.
Leuciscid fishes play a crucial role in riverine ecosystems due to their high abundance, diverse life-history strategies, and specific habitat requirements. The spirlin (Alburnoides bipunctatus) is a rheophilic, lithophilic, and oxyphilic species, highly sensitive to pollution. Because of its strict ecological requirements, it is particularly vulnerable to anthropogenic disturbances, making it a valuable bioindicator of habitat quality in the middle to upper river zones within its distribution range. This paper aims to synthesize existing scientific knowledge on various aspects of spirlin ecology, based on an extensive review of the literature. It addresses key topics such as European distribution, morphology and identification, reproduction and life cycle, diet, movement patterns of both adults and juveniles, and habitat preferences across life stages. Furthermore, it provides an overview of human impacts on the species’ natural ecology and conservation status. A set of key research questions is proposed to stimulate further research and support the development of effective conservation strategies. This review is intended to support researchers in aquatic and fisheries sciences, river managers, and conservation practitioners.
In environments with limited prey resources, coexisting and morphologically similar species that share these resources can compete strongly, potentially resulting in competition and trophic niche displacement. Alternatively, they can partition in their resource use to minimise their competitive interactions. Here, the trophic relationships of two sympatric salmonid fishes, brown trout Salmo trutta and Atlantic salmon Salmo salar, were assessed in two contrasting rivers, a chalk stream where the fish were very fast growing, and an upland stream where the fish were relatively slow growing. Using stable isotope analysis (δ13C and δ15N), the size and position of their stable isotope niches were assessed when the species were sympatric and, in the upland stream, compared between allopatry and sympatry. In both rivers and all sympatric sites, strong patterns of inter-specific stable isotope niche partitioning were evident. In both species in the upland stream, there were only minor differences between their isotope data between allopatry and sympatry, with the position of their isotopic niche similar in both contexts and with overlap in the 95% credible intervals of their isotopic niche sizes. This suggests inter-specific differences in their trophic ecology were driven by differences in functional morphology and habitat use than inter-specific interactions.
Freshwater ecosystems on tropical oceanic islands host unique biodiversity. On Réunion Island, freshwater decapod crustaceans are key indicators of ecosystem health, yet their distribution and conservation status remain poorly documented. This study aimed to investigate the presence of the endemic prawn Macrobrachium hirtimanus, historically reported but unconfirmed for decades, while producing the first island-wide molecular inventory of freshwater decapods using environmental DNA (eDNA) metabarcoding. Water samples (4 L) were collected from 19 riverine sites, and eDNA was extracted and amplified using mitochondrial 16S rRNA primers validated for crustacean detection, followed by high-throughput sequencing. Bioinformatics analyses yielded 250,545 reads and 198 operational taxonomic units (OTUs), representing diverse amphidromous freshwater, coastal marine, and terrestrial decapods. Species richness was largely uniform across sites, though Rivière des Roches, Sainte-Marie, and Saint-Denis hosted higher diversity. Beta diversity showed strong spatial structuring among watersheds, driven primarily by species turnover. Widespread taxa included Atyoida serrata, Caridina typus, and Macrobrachium australe, while C. henriettae and Halocaridinides sp. were more localized. M. hirtimanus was not detected, consistent with its current absence, though continued targeted surveys are required before any formal reassessment. Overall, this study demonstrates the utility of eDNA metabarcoding for monitoring freshwater biodiversity and informing conservation planning on tropical islands.
Remote Underwater Video (RUV) is a promising tool for progressing the future of freshwater fisheries monitoring and management. While uses have previously been focused on marine systems there has been a rise in application for freshwaters. Given the potential for coordinated geographical research using RUVs it is essential that standardised methodologies are described and promoted. We therefore conducted a systematic literature review which returned 185 publications that discussed using RUVs in freshwater environments. These publications used RUVs to measure: abundance, species richness, length-frequency, spawning/mating, behaviour, migration, foraging, size, habitat use, species presence and nesting. There were taxonomic and geographic biases in the results, with commercial salmonid fisheries the primary focus and 49% of published research was performed in North and Central America. While some research has investigated best practices, there are numerous gaps including: determining optimal deployment time in different systems/species compositions, determining suitable acclimation time for behavioural analysis and ascertaining the costs and benefits of using bait as an attractant and stereo-camera for photogrammetry. Until these gaps are addressed, we recommend a cautious set of standards for freshwater RUVs deployment which includes using a standard action camera, recording at ≥30 fps with a resolution of 1080p for 60 minutes. This will ensure that data are broadly comparable between studies. Current bottlenecks in methodology uptake relate to data storage, processing time and cost but this may be overcome with the optimisation of computer vision and machine learning. There are broad opportunities to develop RUV application into a powerful tool for freshwater fisheries management, invasive species detection, and ethological observations if standardised and findability, accessibility, interoperability, and reusability (FAIR) workflows are followed.
Hydropower, utilized for centuries, is promoted globally as renewable energy. The many ecological costs associated with hydropower facilities can be mitigated, but the perceived socio-economic benefits often outweigh environmental concern. The EU Water Framework Directive (WFD), implemented in 2000, established a framework for community action in the field of water policy. It constitutes an instrument to compel hydropower facilities to align with environmental requirements. Based on the WFD and national law, a National Plan for Modern Environmental Conditions for Hydropower (NAP) was formulated in Sweden to renegotiate the environmental permits of around 2000 hydropower plants and dams. The NAP process commenced in 2022 and is estimated to last approximately 20 year. In this review, we assessed the 33 court cases completed until the end of 2024, out of which 22 resulted in permit withdrawal and dam removal, while 11 received decisions requiring remedial measures. The primary focus of remedial measures was re-establishing longitudinal connectivity; other environmental aspects received less attention, and monitoring requirements were almost non-existent. We recommend measures using adaptive design, prioritizing functionality, and monitoring over detailed technical specifications. In addition, greater attention should be given also to aquatic habitats in affected reaches: addressing e.g., flow, water levels, and temperature. In conclusion, this nation-wide process provides a unique opportunity to implement measures that could benefit entire riverine ecosystems.
Macrobrachium hirtimanus (Olivier, 1811) is a species of freshwater shrimp endemic to the Mascarene Islands (Réunion, Mauritius and Rodrigues). The last documented occurrences of this species date from 1980 on Réunion Island and it is thought to be now extinct. In order to determine its taxonomic distinctiveness and conservation status, historical specimens preserved in the collections of the Muséum national d’Histoire naturelle of Paris have been sequenced using a shotgun Next Generation Sequencing (NGS) method, allowing to get mitochondrial DNA barcode sequences (16S, COI) from 4 specimens collected between 1818 and 1926 on Réunion and Mauritius Islands. These sequences will be useful to build reference libraries for future eDNA surveys in the rivers of the Mascarenes that will help ascertain the conservation status of the species by guiding environmental surveys and updating the IUCN assessment. A discussion is provided on reasons leading to the possible extinction of M. hirtimanus as well as a list of specimens extant in museum collections worldwide.
Aquitanian pike, Esox aquitanicus, is a new esocid species described in 2014 and endemic to Southwestern France. The four main goals of the present study were to (i) obtain the first morphological data on the early life stages, (ii) test the effects of temperature on their survival and development, (iii) compare the results with the Northern pike, and (iv) produce a few individuals to exhibit them at the Aquarium of Limoges. Eggs are yellowish, demersal, and sticky. The mean diameter was 2.73 ± 0.08 mm (n = 174). The larval size at hatching was 9.09 ± 0.24 mm (n = 14). The morphological development of 15 larvae was followed during 11 weeks as well as the snout growth in relation to eye size. It appeared that already after three weeks, slight differences between the two species were apparent, and after nine weeks, this snout/eye ratio was twice greater for Northern pike than Aquitanian pike. At the end of the yolk-feeding period, which lasted from 12 days at 18 °C to 36 days at 6 °C, the overall survival rate was over 85% and larvae were of similar size, except at 6 °C. After three months, the best results in term of survival were obtained at 12 °C (60%) and for growth at 15 °C (34.11 mm), followed at 12 °C (27.85 mm). Based on these results, we were able to rear Aquitanian pike for more than 2 years in the Aquarium of Limoges in order to promote the conservation of the species to the public.
Telomeres, repetitive DNA sequences at the ends of chromosomes, are increasingly studied as biomarkers of physiological stress and ageing processes. In ichthyology, fish scales provide valuable information regarding life history and ageing and are thus often preserved in substantial collections. This study adapts a quantitative polymerase chain reaction (qPCR) method to measure relative telomere length (RTL) from archived fish scales. Using the brown trout ( Salmo trutta ) as a model species, we show that archived scales contain enough DNA to perform qPCR-based RTL measurement. RTLs in scales are on average higher than in fins, although we found no significant correlation between RTL values of both tissues at the individual level. Analysing archived scales originating from the subantarctic Kerguelen islands introduced brown trout populations, we find substantial variation in RTL between rivers, a negative relationship with age, and a small positive relationship with body size. We conclude that our method opens up new opportunities to explore stress and aging processes from archived fish scales.
