Biotic and abiotic regulation of a low-density stream-dwelling brown trout (Salmo trutta L.) population: effects on juvenile survival and growth

The effects of biotic (density-dependent) and environmental (flow and temperature) factors on the apparent survival, mean length and size variation of a low-density brown trout population in the juvenile stage, that is, from their first summer (0+) to the end of the second year (1+), were determined. Apparent survival was negatively related to the age class density during the three periods (first summer, first winter and second summer). A significant interaction between the mean flow and 0+ density highlighted a gradient towards strong density dependence acting on fish loss (i.e., mortality or migration) with decreasing summer flow. Conversely, no density dependence was reported at higher mean flows. The mean length was determined by density-dependent and density-independent (temperature and flow) factors throughout the study period. The negative relationship between fish length and intracohort density was highly significant during the three periods. The yearling (1+) density was negatively related to 0+ fish length measured after the first summer, suggesting intercohort effects. A positive effect of temperature on fish length was observed. Mean length after the summer seasons (0+ and 1+ fish) was also positively related to mean flow. Fish size variation around the mean measured with the coefficient of variation (CV) increased with increasing 0+ densities, both at the end of the first summer and the first winter. Results suggested that density-dependent and density-independent factors acted jointly on apparent survival and growth with a predominance of biotic processes. We discussed the potential implications of density-dependent regulations on growth and survival for population resilience after catastrophic events.