Regular, cyclic oscillations in cohort strength in an allopatric population of brown trout. Salmo trutta L.

Dominant year‐classes of brown trout occurred at regular time‐intervals in the alpine Lake Skavatn, Norway. In samples obtained by beach scining, electrofishing and gill‐netting during 1989–1992, yearclasses 1979, 1982, 1985, 1988 and 1991 were much more abundant than their neighbouring year‐classes. Correspondingly, in a sample of gillnetted fish from 1979, the year‐classes 1973 and 1976 were dominant. Spawning areas in the outlet are virtually absent, and lotic rearing areas for juveniles very restricted. Young‐of‐the‐year immigrated to the lake during the autumn, and juveniles inhabited the restricted littoral cobble areas until they reached a length of about 10–12 cm and an age of 3+. Competitive exclusion by dominant year‐classes may cause the regular, cyclic oscillations in cohort strength. An abundant year‐class of juveniles occupying the restricted suitable lacustrine rearing areas may exclude younger fish by inter‐cohort interference. The smaller fish are forced to unsheltered marginal rearing areas where they presumably suffer increased mortality. A new strong year‐class can arise every 3 years when the dominant fish leave the rearing areas.