A comparison of the effects of overhead cover on the growth, survival and haematology of juvenile Atlantic salmon, Salmo salar L., brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson

The provision of floating annular covers to conventional tangential-flow rearing tanks significantly increased the growth rate of underyearling Atlantic salmon during the period July–November. This increased the proportion of potential 1-year-old smolts from 22% to 38% of the population ( P < 0.001). Salmon without access to overhead cover grew slowly and showed haematological signs of a chronic stress response (thrombocytopenia and lymphocytopenia). Mortality rates and the incidence of disease in the young salmon were not influenced by the provision of cover. Overhead cover had no significant effects on any of these parameters in underyearling brown trout and rainbow trout. These findings are discussed in relation to feeding opportunity and behavioural differences between speices.