Size- and density-dependent habitat use in predators:: consequences for habitat shifts in young fish

1. When different size classes of predators use different habitats, the habitat-specific predation risk for prey may change as prey increase in size due to the size-dependency in predator-prey interactions. 2. To study the effects of predation risk on habitat shifts in growing prey, we used a combination of whole-lake manipulations and field data, covering 5 years from three lakes on (1) body sizes at habitat shifts in young-of-the-year perch (YOY) perch (Perca fluviatilis ) and (2) habitat use and densities of two size classes of cannibals (1 year old and greater than or equal to 2 years old). 3. YOY perch shifted habitat to the pelagic immediately after hatching and exhibited a second shift back to the littoral zone at a size range of 11-30 mm. Cannibalism from 1-year-old perch decreased over time, whereas cannibalism from perch greater than or equal to 2 years old increased over time but did not include early larval stages. The habitat use of 1-year-old perch was restricted to the littoral habitat, whereas perch greater than or equal to 2 years old used both the pelagic and the littoral habitats. 4. Habitat shifts in YOY perch were unrelated to differences in resource levels between the two habitats as YOY perch were either yolk sac feeding (first habitat shift) or had similar foraging gains in both habitats (second habitat shift). 5. The size when YOY perch shifted back to the littoral zone was related positively to the densities of 1-year-old perch, whereas no clear relationship was found between shift size and the density of perch 2 greater than or equal to years old. The latter was suggested to be due to the higher density and the larger variation in densities of 1-year-old perch and that the predation risk from perch greater than or equal to 2 years old was less habitat-specific. 6. Our results suggest that size-dependent habitat use and size specific predation risk evoked by cannibals affect the size when YOY perch shift back to the littoral zone.