Data from: High summer macrophyte cover increases abundance, growth and feeding of juvenile Atlantic salmon

Aquatic habitats are severely threatened by human activities. For anadromous species, managing freshwater habitats to maximise production of more, larger juveniles could improve resilience to threats in marine habitats and enhance population viability. In some juvenile salmonid habitats, complexity created by large substrates provides resources and reduces competitive interactions, thereby promoting juvenile production. In lowland rivers, which lack large substrates, aquatic plants might provide similar complexity and enhance fish productivity. To test the influence of aquatic plants on juvenile Atlantic salmon and sympatric brown trout in a lowland river, we directly manipulated cover of the dominant macrophyte, Ranunculus, in nine sites during summer and autumn for two years. We quantified the abundance, site retention and growth of salmon and trout under high, medium and low Ranunculus cover. To investigate the effects of Ranunculus cover on feeding opportunities and interspecific competition, we quantified available prey biomass and body size, fish diet composition, and compared dietary niche overlap. Experimentally increased Ranunculus cover supported higher salmon abundance in summer and autumn, and higher site retention and growth of salmon in summer. Trout abundance and growth were not influenced by Ranunculus cover, but trout site retention doubled in high relative to low cover sites. Despite weak effects of Ranunculus cover on prey availability, salmon and trout inhabiting high cover sites consumed larger prey and a higher biomass of prey. Furthermore, dietary niche overlap was lower in high relative to low cover sites, suggesting that abundant Ranunculus reduced interspecific competition. This field experiment shows that high Ranunculus cover can support more and better growing juvenile salmon, and facilitate foraging and co-existence of sympatric salmonid species. Maintaining or enhancing natural macrophyte cover can be achieved through sympathetic in-river and riparian vegetation management and mitigating pressures on them, such as sediment inputs and low flows, or through planting. Further research should test whether macrophyte cover benefits propagate to subsequent life-stages, particularly juvenile overwintering associated with high mortality. This knowledge, in combination with our findings, would further clarify whether beneficial juvenile habitat can improve the viability of at-risk salmonid populations. Overall, our findings suggest that the aims of river restoration might be achieved through promotion of instream aquatic vegetation.Fish abundance dataset Data corresponding to the analyses of juvenile (0+) salmon and trout abundance as a function of Ranunculus cover (%), explanatory variables describing the experimental design (Year, Month, Block), and explanatory variables representing site- and time-specific habitat characteristics of study sites (Water depth [cm], Proportion of Fast Velocities, Velocity Heterogeneity, Macroinvertebrate biomass [mg]). Fish site retention rate dataset Data corresponding to the analyses of juvenile (0+) salmon and trout site rentention rate (Recaptured fish caught in time t, Total fish tagged in time t-1) as a function of Ranunculus cover (%), explanatory variables describing the experimental design (Year, Period, Block), and explanatory variables representing site- and time-specific habitat characteristics of study sites, averaged across time period (Water depth [cm], Proportion of Fast Velocities, Velocity Heterogeneity, Macroinvertebrate biomass [mg]). Fish growth dataset Data corresponding to the analyses of change in weight (g) of individual juvenile (0+) salmon and trout as a function of Ranunculus cover (%), initial weight (g), and explanatory variables describing the experimental design (Year, Period, Block, Site). Macroinvertebrate biomass surber dataset Data corresponding to the analyses of macroinvertebrate biomass (sum) and average macroinvertebrate size (mean) in Surber samples as a function of Ranunculus cover (%) and explanatory variables describing the experimental design (Year, Month, Block, Site). Fish prey biomass dataset Data corresponding to the analyses of fish prey biomass (sum) and average fish prey size (mean) in individual fish diet contents as a function of Ranunculus cover (%), fish species (Salmon, Trout), individual fork length (cm), and explanatory variables describing the experimental design (Year, Month, Block, Site). Fish diet niche dataset Data corrresponding to the analysis of dietary niche overlap between salmon and trout in different Ranunculus cover levels using proportional abundance and proportional biomass of prey taxa in individual fish diet contents