Microhabitats Chosen by Brown Trout for Feeding and Spawning in Rivers

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Abstract
The water depth, water velocity and substrate used by adult brown trout S. trutta for feeding and spawning in rivers. General hypotheses were: brown trout prefer specific magnitudes of environmental variables and occupy positions through choice; the preferred value of any variable for a particular activity is the same in all rivers; brown trout prefer different values of the same variable for different activities. Surface observation was used to locate 140 feeding and 140 spawning positions used by brown trout in both isolated and sympatric (with rainbow trout S. gairdneri) populations in 6 diverse rivers in New Zealand. Brown trout (mean fork length 42 cm) preferred a mean depth of 65.0 cm and a mean velocity of 26.7 cm .cntdot. s-1 at the position occupied by the fish for feeding, but for spawning they preferred a mean depth of 31.7 cm, a mean velocity of 39.4 cm .cntdot. s-1, and a mean substrate size of 14.0 mm. Analysis of variance showed brown trout preferred the same velocity for the same activity in all rivers and years regardless of whether they were from allopatric or sympatric populations, but microhabitats used for feeding and spawning were significantly different. Velocity appeared to be the most important factor determining position choice but ranking of factors may vary with the type of activity. Brown trout chose positions with optimum combinations of depth and velocity instead of positions with more preferred values of either factor alone. Population size may be limited by the amount of the least abundant activity-specific microhabitat.