Individual variation in habitat use and growth of male and female brown trout

We examined variation in growth and habitat use of individually PIT-tagged brown trout Salmo trutta in three stream enclosures. each divided into a fi ne substrate, deep pool habitat and a coarse substrate, shallow habitat. Ha bitat use and movements of individual fish were monitored continually by pl acing PIT detectors between habitats. All fish were measured and weighed bi weekly over a three month period. There was no significant relationship bet ween habitat use and initial body size, nor was there a consistent relation ship between habitat use and densities of benthic macroinvertebrates or abu ndance of drifting invertebrates in the two habitats. Most habitat changes occurred at night, with activity peaks just prior to sunrise and after suns et. Trout used pools more at night than during the day. Within any given da y, diurnal and nocturnal habitat use of individual fish varied little, with variation greater at night than during the day. Partial habitat segregatio n by sex was observed; only males used pools extensively during daytime, wh ereas males and females used riffles. Growth rate was positively related to use of pools during daytime but not a t night. Growth rate was also affected by enclosure, with growth rates bein g highest in the most downstream enclosure, which had the deepest pool (mea n of 42 cm) and lowest in the most upstream enclosure, which had the shallo west pool (mean of 28 cm). A complete exchange of trout between the most up stream and downstream enclosure indicated that the enclosure effect was due to physical differences and not to individual fish differences between enc losures. The effect appears to have been caused by differences in depth as daytime use of pools was correlated with the area of the pool greater than or equal to 35 cm deep, and production of trout biomass per enclosure was d irectly related to mean pool depth. Our results suggest that there is a rel ationship between habitat use and growth of individuals that is independent of body size, but that this relationship is influenced by sex of the fish and by the physical characteristics of the environment. Further, the data i ndicate that short-term behavioral decisions on habitat use by brown trout have a potential effect on longer-term individual fitness through growth ra tes.