A 27-year study of brown trout population dynamics and exploitation in Lake Songsjoen, central Norway

From 1968-1984 (period I), a brown trout Salmo trutta population in a 70-ha oligotrophic lake in central Norway was exploited using larger mesh gill-n ets selectively removing the larger fish. From 1985-1994 (period II), inter mediate sized fish were removed using smaller-mesh sizes gill-nets. Fishing mortality and CPUE were correlated positively with effort and numbers of f ish >3 years old for period II. The gill-net catchability was correlated ne gatively with spawner biomass and number of trout >3 years old. The signifi cant positive correlation between natural mortality and stock biomass and s pawning stock biomass indicated density-dependent mortality. The significan t correlation between spawning stock and recruitment described by the Bicke r model, indicated density-dependent recruitment of 1-year-old trout. The f ishing regimes in the two periods affected the population dynamics and dens ity differently. Selective removal of smaller fish permitted the larger fis h to survive, and was beneficial in reducing Fish density and maintaining s tocks at low levels, consequently, achieving the expected increase in fish growth rates. (C) 2000 The Fisheries Society of the British Isles.