Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x<overline> = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x<overline> = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

The global decline of freshwater mussels and their crucial ecological services highlight the need to understand their phylogeny, phylogeography and patterns of genetic diversity to guide conservation efforts. Such knowledge is urgently needed for Unio crassus, a highly imperilled species originally widespread throughout Europe and southwest Asia. Recent studies have resurrected several species from synonymy based on mitochondrial data, revealing U. crassus to be a complex of cryptic species. To address long-standing taxonomic uncertainties hindering effective conservation, we integrate morphometric, phylogenetic, and phylogeographic analyses to examine species diversity within the U. crassus complex across its entire range. Phylogenetic analyses were performed using cytochrome c oxidase subunit I (815 specimens from 182 populations) and, for selected specimens, whole mitogenome sequences and Anchored Hybrid Enrichment (AHE) data on ∼ 600 nuclear loci. Mito-nuclear discordance was detected, consistent with mitochondrial DNA gene flow between some species during the Pliocene and Pleistocene. Fossil-calibrated phylogenies based on AHE data support a Mediterranean origin for the U. crassus complex in the Early Miocene. The results of our integrative approach support 12 species in the group: the previously recognised Unio bruguierianus, Unio carneus, Unio crassus, Unio damascensis, Unio ionicus, Unio sesirmensis, and Unio tumidiformis, and the reinstatement of five nominal taxa: Unio desectus stat. rev., Unio gontierii stat. rev., Unio mardinensis stat. rev., Unio nanus stat. rev., and Unio vicarius stat. rev. Morphometric analyses of shell contours reveal important morphospace overlaps among these species, highlighting cryptic, but geographically structured, diversity. The distribution, taxonomy, phylogeography, and conservation of each species are succinctly described.

Fragmentation of rivers by manmade barriers has impeded the ability of riverine fish to move freely. Barriers can be improved by fishways that can partially mitigate the negative impacts by acting as aquatic corridors. Effective fishways require knowledge about the physiological and spatial demands of fish species, but the existing knowledge largely derived from laboratory settings. Evaluating fishway performance is needed for optimisation of their hydraulic design and positioning. Qualitative methods include trapping, electrofishing, and camera observations to estimate the number of individuals passing (effectiveness). For quantitative assessment, the study of individual fish behaviour can identify fishway sections in need of improvement and estimate associated efficiencies. This can be accomplished by telemetric techniques such as PIT tagging, radio, and hydroacoustic telemetry.

This study highlights the conservation problems faced by the tiny freshwater bivalves of the family Sphaeriidae, also known as pea, pill, or fingernail clams (or mussels) in Europe. Despite their global distribution, assumed ecological importance, and potential uses, basic knowledge about their taxonomy, biology, and ecology is very limited and much lower than for the larger freshwater bivalve taxa. Immediate scientific priorities are required to fill knowledge gaps regarding their taxonomy, genetic diversity, distribution, reproductive cycle, ecosystem functions, and population trends. Such fundamental knowledge is necessary to identify specific threats and develop appropriate conservation actions. Deploying environmental DNA analysis at a large scale could be a valuable way to fill gaps in distribution and strengthen monitoring in areas where local taxonomic knowledge is lacking. Until taxon-specific management plans can be developed, we recommend that efforts concentrate on the general protection and restoration of wetland habitats, implementing pollution control measures, and managing invasive species. These actions should be complemented by community engagement through citizen science initiatives. Additionally, prioritising data collection to fill existing knowledge gaps and updating conservation statuses (Red Lists) based on comprehensive assessments will be crucial. Implementing these actions will provide a starting point for the broader protection of freshwater ecosystems, thus benefiting pea clams and other interconnected species within these habitats.

Variation in thermal conditions during embryogenesis can have far-reaching impact throughout ontogeny and may give rise to behavioural variation. Many animals, such as salmonids, exhibit behavioural trade-offs related to foraging and predator avoidance. How embryonic temperature affects these behaviours has remained unexplored. Not only abiotic conditions during embryogenesis but also biotic factors such as predator conditioning may affect fish behaviour, especially anti-predator responses. We examined how elevated temperatures and predator odours throughout embryogenesis affect the behaviour of 28-37 mm young-of-the-year brown trout (Salmo trutta) in encounters with predators, namely Atlantic salmon (Salmo salar; 20 cm) and burbot (Lota lota; 40 cm). Juvenile brown trout were more active and aggressive if they were incubated in warmer water as eggs than if they were incubated in colder water, and trout remained inactive longer when encountering predators if they were cold incubated. Brown trout were less active and aggressive when an Atlantic salmon was present than when a burbot or no predator was present. Behavioural responses did not differ between trout that had been subjected to water with versus without predator odours during embryogenesis, possibly because brown trout were not subjected to conspecific alarm cues during egg incubation. This study shows that thermal conditions during embryogenesis can influence fish behaviour early in life and thus contribute to behavioural variation, with potential effects on life history. Considering the rapid warming of northern regions, elevated embryonic temperatures may contribute substantially to variation in salmonid behaviour in the near future. Variation in environmental conditions during embryogenesis of salmonids can have far-reaching impact throughout ontogeny and may give rise to variation in anti-predator behaviour. In a laboratory experiment, we showed that elevated temperatures throughout embryogenesis increased the activity and aggression of 28-37 mm brown trout fry and reduced the time to first movement in encounters with predators (burbot and Atlantic salmon). Predator odour during embryogenesis did not affect brown trout fry behaviour.image

Freshwater mussels are declining throughout their range. Their important ecological functions along with insufficient levels of natural recruitment have prompted captive breeding for population augmentation and questions about the usefulness and applicability of such measures. This article reviews the current state of captive breeding and rearing programmes for freshwater mussels in Europe. It considers the various species, strategies, and techniques of propagation, as well as the different levels of effort required according to rearing method, highlighting the key factors of success. Within the last 30 years, 46 breeding activities in 16 European countries have been reported, mainly of Margaritifera margaritifera and Unio crassus. Some facilities propagate species that are in a very critical situation, such as Pseudunio auricularius, Unio mancus, and Unio ravoisieri, or multiple species concurrently. In some streams, the number of released captive-bred mussels already exceeds the size of the remaining natural population. Rearing efforts range from highly intensive laboratory incubation to lower intensity methods using in-river mussel cages or silos. Most breeding efforts are funded by national and EU LIFE(+) grants, are well documented, and consider the genetic integrity of the propagated mussels. Limited long-term funding perspectives, the availability of experienced staff, water quality, and feeding/survival during early life stages are seen as the most important challenges. Successful captive breeding programmes need to be combined with restoration of the habitats into which the mussels are released. This work will benefit from an evidence-based approach, knowledge exchange among facilities, and an overall breeding strategy comprising multiple countries and conservation units.

Conditions early in ontogeny can have considerable effects later on in life. Many salmonids spawn during the autumn, and temperature during subsequent embryogenesis may have far-reaching effects on life-history traits, especially when considering ongoing climate change. Even biotic conditions during embryogenesis, such as predation threat, may affect later life stages. Here, we examined how predator odours and increased temperatures affect embryonic growth and development of a fish (brown trout Salmo trutta). We found that embryos had lower body mass and greater yolk volume close to hatching when subjected to predator odours. Trout embryos incubated at temperatures representing natural winter conditions were larger than embryos incubated at higher temperatures, although the latter hatched earlier. Fry sizes at emergence did not differ between treatments, perhaps because of compensatory growth during spring. This study shows that predator presence can have similar effects on embryonic growth of salmonids as warming winters, with possible impact later in ontogeny. 

In the context of the European Union (EU) Drinking Water Directive, freshwater mussels (Order Unionoida: Bivalvia) can help us face the challenges of safe drinking water provisions for all citizens in the EU. Specifically, the implementation of high frequency noninvasive (HFNI) valvometers allows the early detection of eventual pollution events in drinking water treatment plants. Currently real-time behavioral analysis is deployed in a number of EU countries, and we foresee a bright future as new technological advances are developed concerning HFNI valvometers. 

During winter, stream fishes are vulnerable to semi-aquatic predators like mammals and birds and reduce encounters by being active in darkness or under surface ice. Less is known about the behavior of fishes towards instream piscivorous fishes. Here, we examined how surface ice and light affected the anti-predator behavior of juvenile brown trout (Salmo trutta Linnaeus, 1758) in relation to piscivorous burbot (Lota lota Linnaeus, 1758) and northern pike (Esox lucius Linnaeus, 1758) at 4 degrees C in experimental flumes. Trout had lower foraging and swimming activity and spent more time sheltering when predators were present than when absent. In daylight, trout's swimming activity was not affected by predators, whereas in darkness trout were less active when predators were present. Trout consumed more drifting prey during the day when ice was present, and they positioned themselves further upstream when under ice cover, regardless of light conditions. Trout stayed closer to conspecifics under ice, but only in the presence of pike. Piscivorous fishes thus constitute an essential part of the predatory landscape of juvenile trout in winter, and thus loss of ice cover caused by climate warming will likely affect trout's interactions with predators.

Identification of ecosystem services, i.e. the contributions that ecosystems make to human well-being, has proven instrumental in galvanising public and political support for safeguarding biodiversity and its benefits to people. Here we synthesise the global evidence on ecosystem services provided and disrupted by freshwater bivalves, a heterogenous group of >1200 species, including some of the most threatened (in Unionida) and invasive (e.g. Dreissena polymorpha) taxa globally. Our systematic literature review resulted in a data set of 904 records from 69 countries relating to 24 classes of provisioning (N = 189), cultural (N = 491) and regulating (N = 224) services following the Common International Classification of Ecosystem Services (CICES). Prominent ecosystem services included (i) the provisioning of food, materials and medicinal products, (ii) knowledge acquisition (e.g. on water quality, past environments and historical societies), ornamental and other cultural contributions, and (iii) the filtration, sequestration, storage and/or transformation of biological and physico-chemical water properties. About 9% of records provided evidence for the disruption rather than provision of ecosystem services. Synergies and trade-offs of ecosystem services were observed. For instance, water filtration by freshwater bivalves can be beneficial for the cultural service 'biomonitoring', while negatively or positively affecting food consumption or human recreation. Our evidence base spanned a total of 91 genera and 191 species, dominated by Unionida (55% of records, 76% of species), Veneroida (21 and 9%, respectively; mainly Corbicula spp.) and Myoida (20 and 4%, respectively; mainly Dreissena spp.). About one third of records, predominantly from Europe and the Americas, related to species that were non-native to the country of study. The majority of records originated from Asia (35%), with available evidence for 23 CICES classes, as well as Europe (29%) and North America (23%), where research was largely focused on 'biomonitoring'. Whilst the earliest record (from 1949) originated from North America, since 2000, annual output of records has increased rapidly in Asia and Europe. Future research should focus on filling gaps in knowledge in lesser-studied regions, including Africa and South America, and should look to provide a quantitative valuation of the socio-economic costs and benefits of ecosystem services shaped by freshwater bivalves.