Non-additive effects of density-dependent and density-independent factors on brown trout vital rates

Interactions between density-dependent and density-independent processes can lead to variation in both growth and survival rates. Detecting such effects, however, will often require sampling on an individual level and at the appropriate spatial and temporal scale. This study documents substantial variation in survival and growth of stream-dwelling brown trout Salmo trutta from a small Norwegian stream. The data is based on seasonal capture-recaptures of individually marked trout on fixed stations during eight years. The fish were small-sized, rarely reaching sizes larger than 20 cm and ages older than seven years. Density varied between 0.2-0.8 fish m(-2). Variation in survival and recapture probabilities was analysed using program MARK. Apparent survival (the probability of being alive and present within the study area) generally decreased with increasing trout density and increasing drought level (measured as lowest observed water flow) during both winter and summer. Further, there was a significant interaction effect between density and water flow, indicating that density-dependent effects on survival predominated when environmental conditions were benign (no drought), while density-independent processes were most important under harsh environmental conditions (drought). Observed length-at-age during autumn indicated a more or less linear growth trajectory throughout life, and no effect of density, water flow or temperature was found. However, using the individual-based capture-recapture data to estimated specific growth rate, significant positive effects of water flow and temperature and a negative effect of density were identified. Thus, the capture-recapture data suggest a strong potential for population regulation at the rather low densities found in this stream, and regulation may occur both through effects on survival and growth.