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 maximize 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 the 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 or 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 the 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 in-stream aquatic vegetation.