Determinants of habitat use in large roach

In this study gillnet catches of large roach Rutilus rutilus and their potential predators, large perch Perca fluviatilis, pikeperch Sander lucioperca and pike Esox lucius, in combination with data on food resources and abiotic variables were used to reveal the variables influencing the habitat use of large roach in Lake Groser Vatersee. The occurrence of large roach was negatively coupled with the occurrence of potential predators. Although almost all roach studied were larger than those actually fed on by the predators, high roach catches never coincided with high predator catches. Since the overall habitat-specific distribution of roach and its predators was nevertheless highly similar, this indicated that large roach obviously avoided the immediate vicinity of the predators without being forced to a full habitat shift. The behavioural response to predators of roach in Lake Groser Vatersee was clearly length dependent, since the patterns of diel habitat use differed between large and small roach. For lake-wide estimates of trophic interactions, size groups of prey should be separated, since size-dependent trait changes may impact habitat-specific resources in a contrasting way. # 2006 The Fisheries Society of the British Isles