Individual variation and interrelationships between swimming performance, growth rate, and feeding in juvenile rainbow trout (Oncorhynchus mykiss)

Variation among individuals in specific growth rate (SGR), feeding, and two measures of swimming performance and their possible interrelationships were investigated in juvenile rainbow trout (Oncorhynchus mykiss) kept in groups on either satiation or half-satiation rations. Maximum sustainable velocity (U-crit) was measured as an index of aerobic swimming performance and stamina (fatigue time in a fixed-velocity test at 6 body lengths.s(-1)) as an index of anaerobic performance. Individual performance in both of these tests was found to be significantly repeatable. Trout fed on half-satiation ration exhibited significantly lower mean values of SGR and body size and higher levels of aggression-related fin damage, but no significant differences in stamina, relative or absolute U-crit, glycogen content, or plasma cortisol. However, in these fish, there was a significant negative relationship between SGR and relative U-crit, a significant positive relationship between SGR and stamina, and a significant positive relationship between SGR and total daily meal. None of these relationships were seen in fish fed to satiation. Plasma cortisol and tissue glycogen stores were not related to SGR. These results indicate that under the intensified competition of restricted ration, there are trade-offs between growth, feeding, and different types of swimming performance.