The once abundant European eel (Anguilla anguilla) is currently classified as a Critically Endangered species by the International Union for Conservation of Nature. Although both the European Union (through the 2007 Eel Regulation) and its member states have implemented various conservation actions, many uncertainties remain regarding the actual and anticipated effectiveness of these measures. This study aimed to identify and prioritise the conservation and recovery measures for the European eel, using the expert-based Delphi technique. We conducted a two-round Delphi survey on eel conservation measures (these measures were derived from an extensive review of the peer-reviewed literature). The findings highlight the need for establishing a pan-European eel recruitment monitoring programme (to obtain reliable eel population estimates), restoring access to suitable inland habitats by removing migration barriers and improving habitat quality. Experts also emphasized the necessity for a more stringent silver eel escapement target and stronger enforcement of this target. While restocking of eel remains a contentious issue, a set of preconditions could render it conditionally acceptable. This study offers important insights into the areas of consensus and divergence among eel experts in the challenging field of eel conservation. We hope these results strengthen eel management decision-making such that this Critically Endangered species does not become a museum specimen for future generations. 

Anthropogenic structures such as dams, weirs, and hydropower plants constitute adverse obstacles to diadromous fish migrating between their rearing and spawning grounds. During their downstream spawning migration, catadromous European eel (Anguilla anguilla) frequently encounter hydropower plants, often resulting in severe injury or mortality. Accordingly, promoting successful passage is necessary to improve migratory conditions for this critically endangered species. In large-scale flume trials, we used time-to-event analysis to compare the passage rate of eels encountering three types of angled racks with 30 mm bar spacing and bars oriented (i) horizontal and streamwise, (ii) vertical and streamwise, and (iii) vertical and angled. In addition, we investigated the effects of eel individual behavioral type, as measured by distance travelled in an open field test, on passage rate. We found that passage rate was positively affected by individual activity in the open field test, whereas rack type played a less important role. However, the vertical and streamwise rack allowed more eels to pass through the racks (30%), instead of being guided into a bypass, compared to the horizontal and streamwise and vertical and angled racks (9% and 4%, respectively). The results imply that fish passage solutions should be designed to guide as many individuals of different phenotypes as possible to the bypass, but that currently used downstream solutions for silver eel may instead favor more active individuals. 

In regulated rivers, hydropeaking may create abrupt sub-daily changes in discharge that can rapidly alter fish habitat. Although hydropeaking impacts on juvenile salmonids have been widely studied, short-term effects on foraging by early life stages remain poorly documented. We used a laboratory flume experiment to test how a downramping flow regime and substrate heterogeneity affect the foraging performance of brown trout (Salmo trutta) fry. We exposed fry to four treatments, combining two flow conditions (stable vs. downramping) and two substrates (gravel vs. gravel with cobbles). We quantified individual total prey intake, assessed from stomach contents, and drift-feeding activity from video analyses. Our results showed that the flow treatment had no detectable effect on total prey intake, indicating that both stable and rapid flow reductions did not disrupt short-term feeding under an adequate food supply. The number of drift-feeding attempts was similar across all treatments. In contrast, the presence of cobbles increased total prey intake, suggesting that structural complexity enhanced intake by providing hydraulic refuges and additional feeding opportunities. Prey intake showed a general size-dependent pattern, with larger fry consuming more prey. Our findings indicate that adding or maintaining fine-scale substrate complexity is a potent enhancer of fry feeding and should be considered as a restoration measure for habitat mitigation in hydropeaking rivers. 

The European eel (Anguilla anguilla) faces significant challenges during its migratory lifecycle due to anthropogenic habitat fragmentation. Fish passage solutions, such as eel ramps, aim to mitigate such barriers, but their effectiveness varies and may impose selective pressures on eel phenotypes. This study evaluates the impact of ramp design and water flow on the climbing success of juvenile eels, with particular focus on effects of individual eel exploratory phenotypes. Choice between two ramp designs-laterally flat and V-shaped-was evaluated under low (3 L min-1) and high (9 L min-1) flow conditions. The proportion of eels climbing the flat and V-shaped ramps was similar at low flows (36% and 32%, respectively), while a V-shaped ramp led to a higher proportion of climbs than a flat ramp at high flows (30% and 2%, respectively). Additionally, individuals with lower activity scores had a higher probability of climbing. These findings suggest that ramp design influences eel passage efficiency and highlight the potential for unintended selective pressures against high-activity phenotypes. Optimizing eel passage design is crucial to allow upstream and downstream migration and maintaining population diversity. Further studies are needed to assess if upstream migration over multiple eel ramps can affect the migration or phenotypic selection to ensure that passage design does not inadvertently disadvantage climbing success for certain phenotypes within the already threatened eel population.

In regulated rivers, hydropeaking operations have been identified as having significant effects on spawning, growth, reproduction, and behavior of juvenile salmonids. Such effects may reduce survival rates and population viability, but such long-term studies are challenging. Due to the inherent difficulties of conducting large-scale replicated studies in such regulated rivers in situ, river managers require new tools to assess how mitigation measures may affect fish populations. In this study, we used inSALMO 7.3-SD, an individual-based model developed for hydropeaking studies, to investigate the effects of three levels of minimum baseflows (9, 12 and 15 m3/s) and spawning gravel augmentation on the size and number of Atlantic salmon (Salmo salar) and brown trout (S. trutta) outmigrants in the hydropeaking-regulated River Gullspång, Sweden. Our results show that under hydropeaking conditions, higher minimum flows increased outmigrant size, they simultaneously reduced outmigrant numbers, affecting age-1 trout and age-2 individuals of both species. In contrast, gravel augmentation did not affect outmigrant size but generally increased outmigrant production, mainly benefiting age-1 salmon and age-2 trout, although this effect diminished at higher base flow levels. We identified poor body condition and fish predation of juveniles as the primary mortality sources driving the effects on outmigrant production. Our results indicate that the spatial distribution of juveniles was affected by gravel addition and that this effect in turn led to altered mortality rates. Our findings underscore the complex interplay between flow management, habitat modifications, and species-specific behaviors. Also, they highlight the need to carefully balance operational and morphological restoration measures to optimize habitat quality and support effective conservation of salmonids in hydropeaking-regulated rivers. 

The freshwater pearl mussel (Margaritifera margaritifera) is among the most endangered unionids in Europe. Many populations are highly specialized and solely infest specific parapatric populations of either brown trout (Salmo trutta) or Atlantic salmon (Salmo salar). River fragmentation can prevent mussel populations from completing a generational turnover if their migratory hosts can no longer reach upriver mussel habitats. Here, we tested a seminatural reintroduction method and compared the mussel infestation rates on different salmonid ecotypes in an experimental field scenario where previously separated salmonid populations will be reconnected following river rehabilitation. In our study, upriver resident trout sympatric to the mussels and migratory trout and Atlantic salmon allopatric to the mussels (separated by several migration barriers) were cohabitated in in-field cages containing M. margaritifera allowing for a seminatural infestation. We demonstrate that all three salmonid ecotypes hosted a significantly different number of larval mussels: the resident trout was the most heavily infested, while the allopatric salmon hosted the fewest. This study demonstrates that, insofar as only the mussels are moved between locations, in-field cage infestation can be a successful method for mussel conservation as it requires significantly less investment than producing juvenile mussels in a dedicated facility. Further work should be conducted to evaluate the long-term success rate of this reintroduction method and on how to optimize the husbandry of the fish in the cages to minimize mortality. 

Freshwater mussels in the order Unionida are a highly endangered taxon of ecosystem engineers with a variety of conservation and reintroduction efforts around the world attempting to increase wild populations. The unionid lifecycle involves a parasitic larval life-stage (glochidia), primarily adapted for dispersal rather than growth. Little is known about the behavioural effects of the glochidia on their host fish. We investigated the shoaling behaviour and habitat preference of the Eurasian minnow (Phoxinus phoxinus) while infested with the glochidia of one of Europe’s most endangered unionid mussels, the thick-shelled river mussel (Unio crassus). Behavioural assays were carried out in a hydrologically complex flow-through aquaria at two time points of the infestation period, a carrying stage and an excystment stage (14 and 28 days post infestation). We demonstrate that glochidiosis reduces minnow shoaling frequency and increases the number of isolated individuals. However, minnow shoal area and nearest neighbour distance remained unaffected. Infested minnows demonstrated a significant preference for slower flowing and less turbulent habitat. These results suggest that glochidiosis reduces host swimming efficiency, which likely increases predatory pressure on infested minnows. Simultaneously, this preference for calmer habitats may benefit the encysted mussels, as these habitats are better suited for juvenile mussel excystment. Our results highlight the importance of low-flow and low-turbulence habitats for unionid recruitment, as parasitized host individuals tolerate these conditions well.

Parasitic freshwater mussels are endangered ecosystem engineers with an array of impacts on multiple trophic levels and life stages. While the impacts of adult mussels on separate trophic levels have been studied, few have directly tested how adult mussels can impact trophic interactions, or investigated the impacts of the parasitic mussel larvae (glochidia) on such interactions. We present a laboratory study which mimics two-stream substrates for the endangered thick-shelled river mussel (Unio crassus): one dominated by gravel and one by cobbles. First, the preference of a gammarid (Gammarus pulex) for mussel-dominated habitats was tested in the presence/absence of chemical cues from the predator bullhead (Cottus gobio). Second, the preference of bullhead for mussel-dominated habitats was tested under or without glochidia infestation. Third, the effect of infestation on bullhead predation on gammarids was assessed in the presence of adult mussels. Gammarids only significantly preferred mussel habitats in the absence of predator cues, whereas infested bullhead tended to prefer mussel habitats in cobble substrates. The presence of adult mussels only significantly reduced bullhead predation on gammarids in the gravel habitat, whereas infestation did not affect bullhead predation. Despite gammarids not preferring mussel habitat in the presence of predator cues, mussel beds represent valuable habitat to gammarids as mussel presence can facilitate a reduction of predation by bullhead. Infestation did not affect the rate of bullhead predation on gammarids but did attract bullhead to mussel habitat in cobble substrates. Our results suggest that mussel beds may be valuable habitat for both their host fish and the prey of their hosts, attracting both and increasing predator–prey interactions. This study highlights the cross-trophic mechanisms by which multiple life stages of parasitic mussels can impact the interactions of their surrounding benthic community, underscoring their importance as ecosystem engineers.

The size structure of populations is crucial for predator–prey dynamics and ecosystem function. Anthropogenic pressures such as habitat alteration may affect the demography of many species. We investigated the size structure of European perch (Perca fluviatilis) populations in lakes before and after dam removal by using the planned removals of multiple dams in the outlets of boreal lakes in central Sweden as a natural experiment. In five lakes from which we had obtained data both before and after dam removal, removal resulted in a reduced abundance of small perch and an increase in body size of large, piscivorous perch, but there was no major effect on perch biomass. In a second comparison of 22 lakes where we only had access to data from either before or after removal, the effects were not as evident. We suggest that a plausible mechanism explaining a potential effect on perch demography was alterations in the predator–prey dynamics caused by changes in refuge habitat area for small fish. Specifically, dam removal caused a reduction of aquatic plant coverage. Large piscivorous fish are known to play a crucial regulatory role in controlling lake ecosystem function. Thus, we suggest that lake outlet dam removal may induce processes leading to positive effects on fish populations and ecosystem state. Moreover, our study emphasises the importance of before versus after studies to evaluate restoration measures. 

Biomagnification of environmental toxins is influenced by food chain length, which in turn is shaped by habitat connectivity and food web dynamics. Dam removals are increasingly used as restoration measures, yet their role in reducing contaminant exposure has rarely been quantified. We tested if mercury concentration in pike (Esox lucius) muscle tissue was reduced by removals of lake outlet dams in five oligotrophic, boreal lakes, using a before-after design. For pike of average size (50 cm), mean mercury concentration decreased from 843 to 598 mu g Hg per kg muscle tissue following dam removal, which corresponds to a 29% reduction. These results show that dam removal can lower mercury bioaccumulation in apex predators, providing an additional conservation benefit of restoring natural aquatic connectivity, with positive implications for both ecosystem integrity and human health.