HABITAT USE BY THE FISH ASSEMBLAGES OF 2 CHALK STREAMS

Patterns of habitat use by fish assemblages in two chalk streams in so uthern England were examined to identify species preferences with resp ect to major habitat gradients. Both study sites. although differing i n some physical habitat characteristics? mainly channel width, water t emperature and instream cover, could be arranged similarly along a con tinuum extending from erosional to depositional habitats. Twelve fish species were collected from stream surveys conducted during July 1993. The habitat was partitioned into six fish species associations that c ould be assigned to three habitat guilds: depositional (Barbatula barb atula, Gasterosteus aculeatus and Anguilla anguilla), riffle (Leuciscu s leuciscus, Thymullus thymallus and Salmo salar) and generalist (Salm o trutta, Phoxinus phoxinus, Gobio gobio, Cottus gobio, Lampetra plane ri and Pleuronectes flesus). At low to moderate densities the differen t species associations were collected in the same habitat patch, but a t higher densities there was a clear tendency to mutual avoidance. In particular, large trout appeared as strong interactors that tended to exclude other species from their territories. It is hypothesized that the fish assemblage of the Mill Stream and Bere Stream have partitione d the habitat in such a way as to minimize potential competition. (C) 1997 The Fisheries Society of the British Isles.