Per Anders Nilsson

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.

Understanding the genetic structure of keystone species within river networks is essential for effective conservation and management. While population differentiation of anadromous species often occurs between river systems, less research has been conducted on differentiation within rivers with smaller catchment areas. In this study, we investigated the population genetic structure of wild Atlantic salmon (Salmo salar) across the small-scale river Rönne å system in southernmost Sweden using Restriction-site Associated DNA sequencing (RADseq). Although the Admixture analysis did not detect clearly defined genetic clusters, significant pairwise FST values and DAPC revealed emerging population differentiation among the Rönne å tributaries. The observed patterns are consistent with a system characterized by connectivity, where genetic flow is present but can be reduced by behavioral and ecological factors such as spawning homing behavior and selective movements. These findings suggest that, despite overall connectivity, Atlantic salmon populations in the Rönne å catchment area may function as partially independent sub-populations. This highlights the importance of conservation and management strategies in fragmented river systems to consider population genetic structure to support resilient salmon populations under ongoing anthropogenic pressures.

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.

Effective and locally supported river restoration requires attention to the social dimensions of rivers. This paper examines local perceptions of hydropower and restoration in the river Rönne å, an early case under Sweden’s National Plan for Modern Environmental Conditions for Hydropower. A mixed-methods approach, including a questionnaire and qualitative interviews, explores how inhabitants relate to their river and view the removal of three low-production dams, offering one of the most detailed assessments of stakeholder values in a Swedish hydropower river. The findings reveal important tensions between energy production and ecological restoration: While hydropower retains cultural legitimacy, support declines when ecological costs outweigh energy benefits. The river holds strong recreational, cultural, and symbolic meanings, embedding dam removals in broader sociocultural contexts rather than solely technical or ecological. These findings highlight the importance of participatory efforts and governance that align ecological goals with the diverse ways people relate to rivers.

Body size is frequently invoked to explain within-population variation in migratory behaviour because of its far-reaching implications for physiological and ecological processes. Here, we investigated how autumn body size influenced spring river-to-sea migration probability and timing in juvenile brown trout (Salmo trutta) in the river Nybroån, Sweden. To achieve this, we used a combination of individual-based tracking and trapping during two consecutive years. We report a non-linear relationship between autumn body size and probability of spring migration, with migration likelihood increasing steeply up to a body length of ~110 mm before declining with further increasing body size. Among migrants, larger individuals migrated earlier than smaller conspecifics. Our results highlight that autumn body size may be used as a reliable predictor of migratory behaviours in juvenile brown trout and underscore the utility of using recruitment surveys to forecast life-history transitions and suggest that probabilistic reaction norms based on body size can support monitoring and conservation strategies in migratory fish populations.

Background: Natural selection can be a potent contributor to speciation, but how exactly this occurs remains unclear. Studying behavioral isolation and the divergence of mating behaviors among multiple isolated populations can help uncover the importance of divergent adaptation in the evolution of reproductive isolation. Here, we utilize common-garden reared virgin Bahamas mosquitofish ( Gambusia hubbsi ) derived from wild populations adapted to either high- or low-predation risk environments. Results: While we know that parallel evolution of female mate choice has contributed to behavioral isolation between contrasting predation regimes in this radiation, we here discovered that male mating behaviors expressed during one-on-one encounters have also consistently diverged between predation regimes, but have not consequently increased behavioral isolation. On the other hand, we found that female resistance toward a potential mate may represent an underappreciated component of behavioral isolation. Female-to-male aggression during one-on-one interactions was stronger when interacting with foreign males compared to native males, especially males originating from the different predation regime that also elicited female aggression more quickly. Female aggression tended to lead to lower fertilization success, with the lowest fertilization success occurring for mating pairs from different predation regimes. Males displayed their genitalia more toward native females, but performed more copulation attempts toward foreign females—yet, copulation attempts were least likely to lead to genital contact or fertilization when attempting to mate with females from a different predation regime. Conclusions: Overall, behavioral isolation between divergent ecological environments in Bahamas mosquitofish appears to have arisen largely from the actions of one sex. Multiple female mating behaviors contribute to behavioral isolation, while males primarily showed behaviors that seem to attempt to enhance their mating success in the presence of resistant females. Thus, in coercive mating scenarios with indiscriminate males, ecological divergence in traits linked to female mate discrimination can still lead to parallel speciation.

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.

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.

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 μg 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.