Sibling cannibalism in juvenile Asian sea bass (Lates calcarifer) reared under different photoperiods

The effect of the two photoperiods, constant darkness (24H D) and continuous light, (24H L) on the rate of sibling cannibalism in juvenile Asian sea bass (Lates calcarifer Bloch), also known as barramundi, reared indoors under intensive conditions was studied. The indoor experimental setup consisted of two separate systems each having three sixty-liter rectangular rearing tanks connected to a mechanical and a biological water cleaning unit. System 1 was kept under 24 hours of continuous light provided by two 36 watt fluorescent tubes. System 2 was kept under 24 hours of constant darkness except for a short period of 6 minutes during each feeding time. In the course of the study the experimental fish were transferred (after 70 rearing days) to larger rearing tanks of 100 L each. Juvenile Asian sea bass with an average weight of X= 0.73g (64 days post hatching) were used for the 112 day experiment. Each rearing tank was randomly stocked with 187 juveniles. A commercial fish feed (3mm sinking pellets: protein 46%, lipid 12%, fiber 4.6% and ash 8%; was provided to the fish by hand ad libitum four times during the day time. All live individuals in each rearing tank were counted once every two weeks and the rate of cannibalism was determined. Observed dead fish, not caused by cannibalism, were counted and recorded daily as mortality which was calculated. It was found that the accumulated observed mortality (not caused by cannibalism), was similar under 24H D (4.6%) and 24H L (4.1%) conditions. Although feed was provided ad libitum, cannibalism still occurred under both light and dark conditions. The rate of cannibalism, however, was significantly (P < 0.05) lower under constant dark conditions. The results show that a potential predator can swallow a prey up to a maximum of 67% of its own body length. Similar growth performances were observed in the fish groups reared under constant darkness and continuous light. Significantly (p < 0.05), however, highest survival was observed in the groups reared under constant darkness compared with the groups reared under continuous light. Fish reared under continuous darkness were more uniform in size and showed a normal population distribution curve while fish reared under continuous light conditions, showed higher heterogeneity in size and an abnormal distribution of population.