Performance of hatchery-reared brown trout and their effects on wild fish in two small Austrian streams

Two small streams of contrasting physicochemical character, one crystalline and one limestone, were experimentally stocked with brown trout Salmo trut ta. The study design involved doubling (three sites) or tripling (three sit es) the number of large-sized resident fish (>179 or >199 mm total length, dependant on the stream) with an equal mixture of two hatchery strains; thr ee additional sites were left unstocked as controls. In the limestone strea m, short-term survival (3 months) of hatchery fish (both strains) was 80%, compared with 90% for wild fish. In the crystalline stream, survival of hat chery fish was 48% and 62% (dependant on strain), compared with 49% for wil d fish. After 12 months, the survival of hatchery strains declined precipit ously (range: 1-19%), compared with wild fish (range: 13-52%), dependant on stream and strain. After 3 months, about half of the recaptured hatchery f ish were caught outside the 200-m-long sites in which they were stocked. Pe rcent movement of wild fish was affected by stocking density in the limesto ne stream (control, 5%; doubled treatment, 14%; tripled treatment, 20%) but was unrelated to stocking density in the crystalline stream (control 32%; doubled, 20%; tripled, 28%). Stocked strains, on average, lost weight (7-11 %) over the first 3 months in the limestone stream but gained weight (5-25% ) over the same period in the crystalline stream. Growth of wild brown trou t was negatively affected by stocking in the crystalline stream but was una ffected in the limestone stream. Despite the recorded movements, there was no significant change in the population size or biomass of wild brown trout populations due to stocking in either stream.