Effects of grading on individual growth and feeding behaviour of European seabassDicentrarchus labrax

Abstract

Triplicated groups of six small-size (S, 70.3 g), six large-size (L, 96.3 g), and six mixed-size (S + L) fish consisting of three small [S(S + L): 67.5 g] and three large fish [L(S + L): 97.5 g] were reared for 75 days. Fish were individually marked. In all treatments, a feeding hierarchy was established; fish of the highest feeding ranks had the higher mass and specific growth rate (SGR), consumed the majority of food offered with little daily variation, and made more efforts to claim food. Correlation coefficients for S(S + L) and L fish were lower compared to S and L(S + L) fish respectively. The same was observed for the correlation coefficients between mean share of meal and coefficient of daily food consumption variation thus indicating a milder social environment for S(S + L) and L fish. S(S + L) fish had significantly higher SGR, mass gain, and condition factor than S fish. S(S + L) and L fish had a greater percentage of fish consuming the ideal amount of food and obtaining satisfactory feed efficiency than S and L(S + L) fish. Present results indicate that, within the range of body mass examined, grading is beneficial for the large fish but not for the small ones. Grading effects are strongly associated with changes in the strength of social hierarchies.