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 fine substrate, deep pool habitat and a coarse substrate, shallow habitat. Habitat use and movements of individual fish were monitored continually by placing PIT detectors between habitats. All fish were measured and weighed biweekly over a three month period. There was no significant relationship between habitat use and initial body size, nor was there a consistent relationship between habitat use and densities of benthic macroinvertebrates or abundance of drifting invertebrates in the two habitats. Most habitat changes occurred at night, with activity peaks just prior to sunrise and after sunset. Trout used pools more at night than during the day. Within any given day, diurnal and nocturnal habitat use of individual fish varied little, with variation greater at night than during the day. Partial habitat segregation by sex was observed; only males used pools extensively during daytime, whereas males and females used riffles. Growth rate was positively related to use of pools during daytime but not at night. Growth rate was also affected by enclosure, with growth rates being highest in the most downstrean enclosure, which had the deepest pool (mean of 42 cm) and lowest in the most upstream enclosure, which had the shallowest pool (mean of 28 cm). A complete exchange of trout between the most upstream and downstream enclosure indicated that the enclosure effect was due to physical differences and not to individual fish differences between enclosures. The effect appears to have been caused by differences in depth as daytime use of pools was correlated with the area of the pool ≥ 35 cm deep, and production of trout biomass per enclosure was directly related to mean pool depth. Our results suggest that there is a relationship 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 indicate that short-term behavioral decisions on habitat use by brown trout have a potential effect on longer-term individual fitness through growth rates.