The effects of larger fish and size grading on growth performance of white sea bream juveniles (Diplodus sargus)

This study was conducted to determine the effects of size grading on growth performance of white sea bream (Diplodus sargus) juveniles. Wild-caught fish were size sorted into two different groups: small (5.6 g) and large (7.5 g) and then four experimental groups were assigned as 100% small group (S100), 25% large + 75% small group (S75), 50% small + 50% large group (S50), and finally 75% small + 25% large group (S25). Twenty fish were stocked into each tank (450 L) connected with a flow through system in three replicates per treatment. The juveniles were fed to satiation with a commercial feed for 60 days. According to final results, specific growth rate for all periods was similar in all experimental groups (P>0.05). No significant differences were found in survival rate (P>0.05). On the other hand small fish final weight of S50 was significantly higher than the others (P<0.05). There were significant difference between FCR of graded group (S100) and mixed groups (S25, S50 and S75) (P<0.05). The results have demonstrated that the larger individuals in the present study did not suppress growth of the smallest individuals. The fish in S100 were adversely affected by the absence of large fish and this was postulated to be due to poor competition for food acquisition in this group. For this reason size grading seems to be unnecessary to improve growth and survival of white sea bream of around 5.6-7.5 g.