FEEDING TIME, FEED-INTAKE AND GROWTH OF BALTIC SALMON, SALMO-SALAR, AND BROWN TROUT, SALMO-TRUTTA, REARED IN MONOCULTURE AND DUOCULTURE AT CONSTANT LOW-TEMPERATURE

It has previously been reported that the growth of salmonids is improv ed when they are reared in duoculture, possibly due to reductions in t he levels of intraspecific aggression. These claims were examined in m ore detail by studying the time of feeding, feed intake and growth in Baltic salmon, Salmo salar, and brown trout, Salmo trutta, held at con stant low temperature (2.7-3 degrees C) and reared in monoculture or d uoculture for 3 months. Rates of feed intake and growth were initially low, but increased during the course of the experiment, this being pa rticularly evident for the salmon. At the same time, interindividual v ariations in feed intake and growth tended to decrease, and group feed intakes were negatively correlated with interindividual variations in feed intake. Rates of growth of brown trout did not differ significan tly between groups reared in monoculture and duoculture. By contrast, rates of feed intake and growth of Baltic salmon were depressed when t hey were held together with brown trout. Baltic salmon, held together with brown trout, had a significantly greater incidence of fin damage than those held in monoculture, possibly indicating that they were sub ject to aggression from the trout. Fish of both species tended to feed during the hours of daylight, and there was no evidence of an increas e in nocturnal feeding activity amongst fish of either species when he ld in duoculture. (C) 1998 Elsevier Science B.V. All rights reserved.