Effects of group membership and size distribution within a group on growthrates of juvenile sablefish Anoplopoma fimbria

Group membership can confer both advantages and disadvantages to growth in juvenile fishes. The balance between costs and benefits of social interacti ons can shift depending on such factors as the composition of the group (de nsity and size disparity) and the availability of food. We examined the eff ect of these factors on absolute growth and growth depensation in juvenile sablefish, Anoplopoma fimbria. Increasing density and increasing size dispa rity had little influence on absolute growth rates of juvenile sablefish an d the effects of these social factors were not modified by ration level. In experiments testing density effects, absolute growth did not differ among groups of 1, 3, or 10 fish held at high rations, but at low rations single fish exhibited a different pattern of size-dependent growth compared to fis h in groups. In experiments testing disparity effects, absolute growth did not differ between groups with an even size distribution and groups with a mixed size distribution. The relative size of an individual within a group, i.e., small, medium, or large, also did not modify growth, despite evidenc e of higher chasing behavior in mixed size distributions. Although the grow th of small fish was not diminished in the presence of large fish, negative impacts of size disparity were expressed in high levels of cannibalism, wh ich occurred in 42% of groups with a mixed size distribution. Significant g rowth depensation over time occurred in the density experiment, but not in the size disparity experiment, possibly due to the shorter duration of the latter experiment. We suggest that growth depensation was generated by indi vidual variability in growth capacity rather than social effects on growth rates. Schooling behavior, measured by group cohesion indices, increased wi th fish size and was higher in groups with an even vs. a mixed size distrib ution. These results for sablefish are consistent with other schooling spec ies in which growth variability is determined by exploitative competition a nd/or genetic variability in growth capacity rather than interference compe tition.