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Habitat fragmentation may cut off anadromous salmonids from parts of their potential native habitat and separate previously connected populations. Understanding the consequences of this is vital for fish management and prioritization of restoration activities. Here, we show that there is a significant difference in the body morphology, physiological stress response, and aspects contributing to aerobic capacity between juvenile anadromous brown trout, Salmo trutta, collected at a downstream site and an upstream site, separated by 2 km and several challenging stream sections, in a small unfragmented stream system in western Sweden. Following a standardized stress test, there were significant differences between fish from the upstream and downstream sites (plasma cortisol concentration, plasma osmolality, hematocrit, hemoglobin concentration, and mean corpuscular hemoglobin concentration). Plasma glucose concentration did not significantly differ between fish from the two sites. Fish from the upstream site had larger spleen mass, although there was no evidence of differences in ventricle mass or proportion of compact ventricular myocardium. These physiological differences indicate local variation in stress response and highlight the importance of considering local trait variation in river management. If a section of the river becomes fragmented or degraded, and there are differences in the juveniles in different parts of the river, the consequence for the population might be larger than the proportional loss of habitat. [ABSTRACT FROM AUTHOR]

Habitat complexity describes a wide array of spatial distribution patterns of physical structures in habitats. It affects aquatic ecosystems on multiple levels from individuals (e.g., foraging behavior) to species interactions (e.g., predation, prey selection) and entire communities (e.g., biodiversity, food web structure). We present a conceptual framework to classify these effects and use it to summarize recent advances in the field. We identify three main research gaps and limitations preventing a full synthesis of the effects of habitat complexity on aquatic communities and ecosystems. Habitat complexity is often characterized using ad hoc measures, which limits cross‐experimental comparison and meta‐analytical and modeling approaches. The effects of habitat complexity on communities and ecosystems can also involve feedback loops on lower levels of organization including the habitat complexity itself. Such ecological feedbacks can influence habitat formation and amplify or mitigate the direct effects of habitat loss and simplification or habitat restoration on populations and communities, yet are surprisingly little understood. Finally, most studies examine habitat complexity on the presence‐absence scale. This limits our ability to recognize nonlinear responses across habitat complexity gradients, which occur in many contexts in aquatic habitats. Since nonlinear responses can stabilize or destabilize population and community dynamics, we call for the use of a higher resolution of habitat complexity in future studies. We conclude that currently degraded habitats offer exciting opportunities for combining restorative efforts with research that could combine multi‐level experiments and monitoring to improve our understanding of the role of habitat complexity across aquatic ecosystems. This article is categorized under:Water and Life > Nature of Freshwater EcosystemsWater and Life > Conservation, Management, and Awareness [ABSTRACT FROM AUTHOR]

The salmonid fish Brown trout is iconic as a model for the application of conservation genetics to understand and manage local interspecific variation. However, there is still scant information about relationships between local and large‐scale population structure, and to what extent geographical and environmental variables are associated with barriers to gene flow. We used information from 3,782 mapped SNPs developed for the present study and conducted outlier tests and gene–environment association (GEA) analyses in order to examine drivers of population structure. Analyses comprised >2,600 fish from 72 riverine populations spanning a central part of the species' distribution in northern Europe. We report hitherto unidentified genetic breaks in population structure, indicating strong barriers to gene flow. GEA loci were widely spread across genomic regions and showed correlations with climatic, abiotic and geographical parameters. In some cases, individual loci showed consistent GEA across the geographical regions Britain, Europe and Scandinavia. In other cases, correlations were observed only within a sub‐set of regions, suggesting that locus‐specific variation was associated with local processes. A paired‐population sampling design allowed us to evaluate sampling effects on detection of outlier loci and GEA. Two widely applied methods for outlier detection (pcadapt and bayescan) showed low overlap in loci identified as statistical outliers across sub‐sets of data. Two GEA analytical approaches (LFMM and RDA) showed good correspondence concerning loci associated with specific variables, but LFMM identified five times more statistically significant associations than RDA. Our results emphasize the importance of carefully considering the statistical methods applied for the hypotheses being tested in outlier analysis. Sampling design may have lower impact on results if the objective is to identify GEA loci and their population distribution. Our study provides new insights into trout populations, and results have direct management implications in serving as a tool for identification of conservation units. [ABSTRACT FROM AUTHOR]

The life cycle of the endangered freshwater pearl mussel ( Margaritifera margaritifera) includes a parasitic larval phase (glochidia) on the gills of a salmonid host. Glochidia encystment has been shown to affect both swimming ability and prey capture success of brown trout ( Salmo trutta), which suggests possible fitness consequences for host fish. To further investigate the relationship between glochidia encystment and behavioural parameters in brown trout, pairs ( n = 14) of wild-caught trout (infested vs. uninfested) were allowed to drift feed in large stream aquaria and foraging success, activity, agonistic behaviour and fish coloration were observed. No differences were found between infested and uninfested fish except for in coloration, where infested fish were significantly darker than uninfested fish. Glochidia load per fish varied from one to several hundred glochidia, however, and high loads had significant effects on foraging, activity and behaviour. Trout with high glochidia loads captured less prey, were less active and showed more subordinate behaviour than did fish with lower loads. Heavy glochidia loads therefore may negatively influence host fitness due to reduced competitive ability. These findings have implications not only for management of mussel populations in the streams, but also for captive breeding programmes which perhaps should avoid high infestation rates. Thus, low levels of infestation on host fish which do not affect trout behaviour but maintains mussel populations may be optimal in these cases. [ABSTRACT FROM AUTHOR]

1. Parasite-host interactions can involve strong reciprocal selection pressure, and may lead to locally adapted specializations. The highly threatened unionoid mussels are temporary parasites on fish, but local adaptation has not yet been investigated for many species. 2. Patterns of local adaptation of one of Europe0s most threatened unionoids, the thick-shelled river mussel (Unio crassus) were investigated. Eurasian minnows (Phoxinus phoxinus) from two rivers (separate drainage areas) were cross-infested in the laboratory with sympatric and allopatric mussel larvae, while bullheads (Cottus gobio), inhabiting only one of the rivers, were infested with sympatric or allopatric mussel larvae. Larval encystment, juvenile mussel excystment and survival were measured. 3. For one river, but not the other, juvenile excystment from P. phoxinus was highest when infested with sympatric mussels. The opposite pattern was found for C. gobio in this river, where juvenile excystment and post-parasitic juvenile survival from allopatric C. gobio were highest. The results thus cannot confirm local adaptation of U. crassus to P. phoxinus in the study rivers, as excystment was not consistently higher in all sympatric mussel-host combinations, whereas there were potential maladaptive signs of U. crassus in relation to C. gobio. There was no loss of encysted larvae 3 days after infestation until juvenile excystment. Most juveniles were excysted between 17 and 29 days after infestation, and the numbers of excysted juveniles increased with fish size. 4. The results have implications for parasite-host ecology and conservation management with regard to unionoid propagation and re-introduction. This includes the need to (1) test suitability and adaptation patterns between U. crassus and multiple host fish species, (2) evaluate the suitability of certain unionoids and host fish strains after more than 3 days, and (3) determine whether large fish produce more juvenile mussels than smaller fish. [ABSTRACT FROM AUTHOR]

In a field experiment, we examined the effects of structural complexity in the form of added artificial plastic plants and shredded plastic bags on growth and abundance of juvenile brown trout ( Salmo trutta). Just after emergence, the added complexity had a positive effect on the density, biomass and condition factor of young-of-the-year (0+) brown trout. This difference in density was not present six weeks later. In contrast, both young-of-the-year and older brown trout generally tended to be larger in the simple habitat. Hence, our data suggest that increased complexity initially is beneficial for young-of-the-year individuals probably due to lower risk of predation and increased densities of prey. However, as density increases in the complex environment, it may induce negative density-dependent effects, here reflected in smaller sized fish in the complex environment. This might force fish to redistribute to habitats with lower densities of conspecifics as they grow larger. We propose that habitat complexity can increase survival of yearlings in early phases and thereby also affect the overall population structure of brown trout in natural streams. [ABSTRACT FROM AUTHOR]

Juvenile brown trout were experimentally displaced 400 m both upstream and downstream, whereupon growth and homing behavior were examined for three months. No obvious signs of homing behavior were detected, no matter what the direction of displacement was, although an increased activity in displaced fish was detected. Our findings generally support the restricted movement paradigm, at least under favorable habitat conditions and low densities. [ABSTRACT FROM AUTHOR]

Several environmental cues have been suggested to govern the initiation of the downstream migration of smolts (e.g. light, temperature and discharge). Here we investigated the effect of these cues on the initiation of the downstream migration in anadromous brown trout and Atlantic salmon in a small river on the Swedish west coast during two consecutive years. The total smolt production was assessed using a smolt trap and by modelling data from standardized electrofishing. The results show that it is possible to estimate smolt numbers using electrofishing, but that the model had a tendency to overestimate the number of smolts from electrofishing as compared with that from captures with the smolt trap. Discharge had the greatest effect on downstream migration, but temperature was also important when there was no increase in discharge. These results reveal that discharge and temperature govern the initiation of downstream migration but their effect may depend on the amount of precipitation. [ABSTRACT FROM AUTHOR]