MICROHABITAT USE BY STREAM-RESIDENT BROWN TROUT - BIOENERGETIC CONSEQUENCES

Seasonal shifts in the microhabitats used by feeding brown trout Salmo trutia in a northern Spanish river were related to changes in habitat availability. At higher flows, mean water column velocities, focal ve locities, and depths occupied by brown trout were greater and focal el evations above the substratum were less, but no changes in substratum use were detected. Cover use differed between dates, apparently accord ing to the presence of suitable materials. In May, a lower proportion of the observed fish were holding feeding positions than in July, Octo ber, or January. Total metabolic costs associated with holding feeding stations were primarily determined by swimming costs, and both costs varied substantially between dates, mainly as a consequence of the dif ferent swimming speeds of brown trout on the different dates. Standard metabolism was less important than swimming cost in total energy expe nditure and showed no obvious relationship with total metabolic mte. B rown trout exhibited considerable individual variation in swimming cos ts, which seemed not to be related to fish size.