The importance of Ranunculus spp. for juvenile salmonids in lowland rivers

PhD ThesisMacrophytes could be integral to structuring lowland rivers and providing habitat for juvenile salmonids, but currently there is a lack of quantitative evidence evaluating this role. The aim of this thesis was to quantify the influence of a lowland macrophyte, Ranunculus, on juvenile salmonids and their environments in lowland rivers. A catchment-wide correlation study determined salmon and trout densities to be positively associated with Ranunculus cover, and velocity heterogeneity, respectively (Chapter 2). I subsequently implemented a spatially-and-temporally replicated Ranunculus manipulation experiment in a lowland river carrier. High Ranunculus cover supported higher abundances of salmon and trout throughout the summer feeding period. I also identified a strong influence of Ranunculus at the beginning of summer, when salmon abundances, site fidelity and increase in body length were highest (Chapter 3). Focusing on the beginning of summer, I then showed that higher total biomass of prey and a larger average size of prey were found in diets of salmon and trout captured in high Ranunculus cover, suggesting that Ranunculus was facilitating better foraging opportunities. Additionally, the dietary niche overlap between the two species was lowest in high cover, suggesting that abundance of Ranunculus reduced interspecific competition. There was a positive effect of Ranunculus on the growth rates of salmon, but an effect on trout growth was not detected (Chapter 4). Finally, I evaluated direct and indirect effects of Ranunculus on salmon abundance and growth rates (Chapter 5). I showed that salmon abundance was most influenced by the cover supplied by Ranunculus directly, but that this effect was mediated through negative influences of water depths and velocities. The positive influence of Ranunculus on salmon growth rates, however, was more intricately linked to changes in physical habitat and prey resources driven by Ranunculus, than to a direct source of cover. Together, these findings demonstrate a holistic role of Ranunculus in a lowland river, and its potential to improve the population viability of at-risk salmonid population