EFFECTS OF SIZE GRADING ON GROWTH AND SURVIVAL OF JUVENILE TURBOT AT 2 TEMPERATURES

Juvenile turbot were size graded into three size groups (mean initial size): Small (3.4 g), medium (7.0 g) and large (10.5 g), and additiona l fish were held in ungraded (6.6 g) groups. Subgroups (n = 396) of fi sh were tagged and individual growth rates and social interactions wit hin different size categories were studied in fish reared at 13 and 19 degrees C. Size grading of juvenile turbot did not improve growth. Sp ecific growth rates (SGR) were mainly size-related, and no differences in SGR or mortality between the experimental groups at both temperatu res were found. A higher level of social interactions was indicated am ongst medium-sized fish than amongst those in the smallest and largest categories. Excess feeding may have been important factors in reducin g aggression, so that the growth of the smallest individuals was not s uppressed by the larger individuals in the present study. Grading seem s to be an unnecessary operation to improve the growth and survival of juvenile turbot (5-150 g). However, as it was mainly the smallest ind ividuals in each group that died, grading of very small turbot (2-5 g) can be recommended.