Density-dependent growth in brown trout:: effects of introducing wild and hatchery fish

1. Although it is not clear to what extent density dependence acts on the survival, emigration or growth of organisms, experiments testing alternative explanations are rare. A field experiment on 1-year-old brown trout (Salmo trutta L.) was undertaken to address the following questions: are the mortality, movement and growth of wild stream-living trout affected by population density? If so, are the density-dependent effects of released hatchery trout different from the effects of wild fish? 2. In each of two small streams, two replicate treatment blocks were used, each with four treatments assigned to stream sections 50-70 m in length: (1) control, no fish was introduced and population density was kept at its original level. (2) Trout biomass was doubled by introducing additional wild fish. (3) Trout biomass was doubled by introducing additional hatchery fish. (4) Hatchery trout were introduced, but biomass was kept at its original level by the removal of some resident wild fish. 3. We found no treatment effects on the recapture rates of resident trout, which suggests that survival was not strongly affected by competition. They were also remarkably stationary, regardless of treatment. However, trout growth rate was reduced to the same extent in both treatments with increased density, suggesting that growth was negatively density-dependent, and that the density-dependent effects of hatchery trout and introduced wild fish were similar. 4. Wild resident fish grew faster than introduced wild trout, which in turn grew faster than hatchery trout. Hatchery fish and introduced wild fish moved more than wild resident fish. 5. The results show that population density affected growth in trout parr. We conclude that competition is not limited to the underyearlings, as has previously been suggested, and that density-dependent growth is the main density-dependent response in yearling trout. Furthermore, this effect was the same for wild and hatchery-reared competitors, suggesting that stocking of hatchery fish may affect natural populations negatively through density dependence.