Density of juvenile brown trout and Atlantic salmon in natural and man-natural riverine habitats

The density of juvenile brown trout (Salmo trutta L.) and Atlantic salmon (Salmon salar L.) was significantly higher along river bank areas protected against erosion than along natural river banks in the River Gaula, Central Norway. A habitat shift appeared in Atlantic salmon, and a behavioural shift was demonstrated by brown trout from August to October. The effect of habitat on densities of juvenile salmonids should be taken into account as mitigation measures on eroded river banks and when accessing fish production in rivers

Long‐term variation in piscivory in a brown trout population: effect of changes in available prey organisms

The piscivorous behaviour in a brown trout (Salmo trutta L.) population was studied in four discrete periods over seven decades (1917–94) in the hydroelectric reservoir Tunhovdfjord in Norway established in 1919. Piscivorous brown trout were extremely scarce prior to the introduction of two fish species Arctic charr (Salvelinus alpinus L.) and European minnow (Phoxinus phoxinus L.) in the 1920s. Brown trout started eating minnow at 17 cm and Arctic charr at 22 cm of length. In the 1950s, the brown trout predated extensively (60% of analysed trout) on Arctic charr and minnow. During the next four decades, the incidence of piscivorous brown trout declined to 15%, whereas the frequency of brown trout eating Arctic charr remained constant at 10%. The growth pattern, expressed as back‐calculated length, demonstrated similarity in three periods (1920s, 1960s and 1990s) and improved growth in the 1950s. The improvement was addressed the impoundment of a reservoir upstream. We did not find any marked change in growth rate due to piscivority, but coefficient of variance of back‐calculated lengths indicated significant variation in individual growth in age group ≥6 years from 1950 onwards. We accredit this variation to the rise of piscivorous brown trout