Environmental variability and population dynamics of juvenile brown trout (Salmo trutta) in an upstream and downstream reach of a small New Zealand river

Abstract

Population densities, size and condition factor of juvenile brown trout and variation in environmental factors (discharge, temperature, and food availability) were examined over 2.5 years (28 months) in an upstream and downstream reach of Silverstream, a lowland river in New Zealand. Population densities of juvenile brown trout varied between the upstream and downstream reach, with the latter showing considerably higher temporal variation in fish density than the upstream reach. Juvenile brown trout from the downstream reach were larger and had higher condition factors than those from the upstream reach over the study period. Discharge had a negative effect on fish densities in 1 year in the downstream reach, and high water temperatures were recorded in one summer, with possible negative effects on fish densities. However, the different effects of environmental variables on populations between the two reaches could not explain the higher temporal variation in fish densities in the downstream reach as spatial variation in population densities also occurred in years without high discharge events or high summer water temperatures. Variability in food availability over time was higher in the upstream reach, but differences between the two reaches were small. These results suggest that factors other than condition of the local environment are important in controlling brown trout population dynamics in Silverstream.