Effect of depth and substrate on use of stream pools by brown trout

We tested how depth and cobble clusters affected pool use by brown trout Sa lmo trutta in four enclosures in a seminatural, outdoor stream channel. Int o each enclosure, which consisted of a shallow, cobble-filled habitat and a deeper pool, we stocked eight brown trout 9-19 cm long that had been tagge d with passive integrated transponders (PIT). Receiver antennae for the PIT tags were placed between the two habitats to allow continuous monitoring o f movements of individual fish. We found that trout used pools to a greater extent at night than during the day. Alterations of depth and substrate ha d no effect on pool use at night. During the day. increasing water depth ei ther alone or together with adding cobbles increased pool use, whereas cobb le addition alone had no effect on pool use. We suggest that evaluations of physical habitat modifications: in streams should include measurements of population responses as well as information on how, when. and which fish us e newly created habitat structures.