Ecology of stocked brown trout in boreal lakes

Abstract

Ecology of stocked brown trout (Salmo trutta L.) was studied in three boreal lakes, north-eastern Finland. The field experiment was designed so that sources of variation from genetics and treatment at the hatchery of trout were kept minimal. In addition, the study lakes were similar with respect to area, depth, water quality, resident fish species composition, fishing effort and stocking rate. Hence, feeding, energetics and yield of stocked trout were hypothesised to be affected by seasonal and interannual fluctuation in abundance and size of vendace (Coregonus albula L.). Another large field data was collected to study relationship between larval mortality and subsequent recruitment of vendace. Stocking success of brown trout, in terms of growth and net relative yield, was tightly dependent on the proportion of small fish prey in the diet of trout. When the trout fed strongly on small (< 4 g) fish prey, such as vendace, nine-spined stickleback (Pungitius pungitius L.), and perch (Perea fiuviatilis L), their growth was fast, and they also increased storage lipids. When the trout fed mainly on invertebrates their growth was poor, and a strong depletion in storage lipids during the first growing season probably increased owerwinter mortality. Size of trout at release and season of stocking were less significant factors, except the trout released at post-smolt stage showed the poorest success independently of the quality of the foraging environment. Variation in the vendace recruitment was found to be caused mainly by variation in larval mortality during the three weeks after ice-break. Predatorprey interaction between trout and vendace was sensitive to prey growth, because growth of vendace caused a diet shift from vendace to stickleback. At the individual level, growth rate of the trout at the hatchery predicted poorly growth rate in the lakes, suggesting that growth was determined by environment-specific set of traits.