Food- and habitat-segregation in sympatric grayling and brown trout

Grayling (Thymallus thymallus) in the large reservoir Aursjoeen, Norway, did not use the pelagic habitat and only trout (Salmo trutta) larger than 187 mm and older than 4+ were caught there. Both species preferred the upper 8 m in the benthic habitat, but grayling were caught deeper than trout in August and September. Grayling were more closely associated with soft substratum than trout. Association with soft substratum decreased with increasing size in grayling, whereas this relationship was reversed in trout. A low number of empty stomachs indicated a low degree of food competition, but despite this the two species segregated in different key prey items-probably due to differences in mouth shapes. The key prey item for grayling was the benthic Eurycercus lamellatus, with zooplankton dominating the diet of the smallest individuals. Large trout (>300 mm) preferred the benthic Lepidurus arcticus, whereas medium trout (160-300 mm) ate predominantly the limnic-pelagic Bythotrephes longimanus, irrespective of habitat. The distribution of key prey items seemed to be the major factor controlling the habitat use, except in small trout which showed a generalist feeding pattern throughout the period. A generalist feeding pattern, a close association to coarse substratum and absence from the pelagic habitat indicated that small trout were negatively influenced by large individuals. Small grayling seemed less affected by aggressive encounters as they were less associated with coarse substratum and ate predominantly high-risk food such as zooplankton. These individuals probably did shoal in order to reduce predation risk.