Spawning of anadromous rainbow trout, Oncorhynchus mykiss (Walbaum): a threat to sea trout, Salmo trutta L., populations?

Interference competition for territorial space and food resources between p rogeny of anadromous brown trout, i.e., sea trout, Salmo trutta L., and non -indigenous anadromous rainbow trout, Oncorhynchus mykiss, was studied in s tream channels under laboratory conditions. In addition, the two anadromous species were compared by measurements of fecundity, egg size, alevin size, growth rate, and timing of spawning migration. Competition was mediated th rough migration of sea trout parr out of the stream channels and the result s suggest a negative influence on the sea trout in terms of displacement fu rther downstream in the channels and lowered specific growth rate when fish of equal size were tested together Given the size advantages normally foun d, caused by differences in spawning period, sea trout parr held their posi tions in the stream channels while the smaller rainbow trout parr moved out of the system or stayed in areas where sea trout were absent. This study was initiated because an increasing number of ascending rainbow trout was reported in many streams on Gotland. Spawning rainbow trout have been observed to have destroyed sea trout spawning redds. Despite rainbow t rout spawning in many streams, only one descendant from these anadromous ra inbow trout has been found to date. (C) 1999 Elsevier Science B.V. All righ ts reserved.