Effects of size distribution on social interactions and growth of juvenile black rockfish (Sebastes schlegelii)

Growth heterogeneity in rearing groups may complicate feeding operations and inducing cannibalism in cultured fish. To reduce these problems size grading has become a common rearing practice. To better understand the relationship between size grading and growth variation in cultured juvenile black rockfish (Sebastes schlegelii) we studied how size heterogeneity affected their growth performance (body weight, SGR, CVw), social interactions (feeding position, aggressive behavior) and endocrine responses (cortisol, growth hormone, 5-hydroxytryptamine). A batch of sibling fish (0.29-1.73 g) were selected and divided into three groups according to their similarity in size. Three types of size-structured groups were established each type containing two treatments with different fish numbers (n = 30 or n = 6): Small fish alone, small and medium fish mixed, small and large fish mixed. The experiment lasted for 30 days. In all treatments, regardless of the number of fish in the group, the growth rates of small-sized fish in size-homogeneous groups were lower than in size-heterogeneous groups. Consequently, size variation decreased significantly with time in size-heterogeneous groups whereas the opposite pattern was found in size-homogenous groups. Overall, growth heterogeneity tended to stabilize with time in all experimental groups. In the size-heterogeneous groups larger fish always occupied the central feeding area. Outside feeding period small fish avoided the central area, likely to avoid cannibalism, but when food was present small fish practiced a sneaky feeding activity. In general, aggressive interactions between small-sized fish were fiercer when larger fish were absent, and within size-heterogeneous groups aggressive interactions were more common between similar-sized larger individuals than between individuals of different size. The physiological analyses supported the growth and behavioural data. Cortisol levels were lower in the most size-heterogeneous groups than in the more homogenous groups whereas the growth hormone levels in the size-homogeneous group were significantly lower than in the more heterogeneous groups. In conclusion, the growth variation in juvenile black rockfish seems to be a consequence of intensive competition between similar-sized individuals rather than growth suppression of smaller in a size-related dominance hierarchy. Thus a size-grading practice where rearing groups retain an acceptable level of size heterogeneity that limits competition and cannibalism would produce the overall best result in terms of growth and survival.