Habitat use in fish communities

Through the influence of abiotic factors, the habitat use of organisms affects their metabolism as well as other species- and size-dependent individual-based rates. The habitat-specific performances of individuals interacting in different habitats thereby affect biotic interactions. Habitat use is thus central for the outcomes of biotic interactions that, in turn, regulate populations and communities. My aim is to investigate how individual processes are influenced by habitat-dependent abiotic factors, affecting biotic interactions to regulate habitat use and population structures in fish communities. I examined patterns of habitat distribution and population structures of perch (Perca fluviatilis L.), roach (Rutilus rutilus (L.)), and the zooplankton specialist vendace (Coregonus albula (L.)) using a database of standardised test fishing data in lakes. To clarify mechanisms, I experimentally studied predation from perch in pond enclosures as well as relative foraging abilities of the two competitors roach and vendace in aquaria with different temperature and light treatments. To test mechanisms in natural situations, I calculated species- and size-dependent net energy intake, incorporating temperature- and light-dependence, including metabolism, using field data from different habitats in lakes with and without vendace. I also developed and applied a stage-structured biomass model, considering a cold water species (vendace) using two habitats differing in temperature. I thereby studied how climate warming which acts differently on different lake habitats affected temperature-dependent individual-based processes, and results on the population level. Through multi-species studies, I found that a combination of size- and environment-dependent individual processes determining energy gain, rather than predation risk, could explain size- and species-specific habitat use. The single-species study showed that stage-specific intake rates in one habitat, altered by increased temperature, affected intraspecific competition in both habitats, through a mechanism of ‘inter-habitat subsidies’ which altered population structure through maturation and reproduction rates. My thesis shows how including size- and environment-dependent individual processes, and interactions across habitats, increases our understanding of population and community structure as well as effects of environmental change