Effect of stocking density on growth, digestive enzyme activity and cortisol level in larvae and juveniles of Japanese flounder, Paralichthys olivaceus

The aim of this work was to evaluate the effect of stocking density on growth, digestive enzyme activity and cortisol level in larvae (2–35 DAH) and juveniles (46–65 DAH) of Japanese flounder. The body weight and body length were recorded, enzyme activities (trypsin and lipase) and cortisol levels were examined from frozen samples anesthetized with 0.1% MS 222.In the first experiment using larvae, two different densities (1000 and 5000 individuals in triplicate 100 l tanks) were examined.The weight and total length of the low density group was significantly larger (P < 0.05) than that of high density group after 15 DAH. No significant differences (P > 0.05) were found in cortisol concentrations before 30 DAH, while a significantly higher concentration (P < 0.05) was found in the high density group on 35 DAH. No differences were found in digestive enzyme activity.In the second experiment, juveniles of 46 DAH were kept in 100 l triplicate tanks at two different densities (200 and 2000 larvae in 100 l tanks, bottom area 0.22 m2), and reared for 15 days. Bottom settled fish and swimming fish were separately sampled. On 61 DAH, the fish swimming were selectively captured in the high density tanks and 100 individuals were transferred to 30 l tanks and further reared for 4 days. The growth in total length and weight of swimming juvenile flounder was significantly lower (P < 0.05) comparing with settled fish. Growth in total length at 61 DAH was significantly higher (P < 0.05) in the high density bottom settled fish, compared with the low density one. Swimming fish transferred from high density tanks, showed a significant increase (P < 0.05) in body weight 4 days after the transfer. Transferred fish also showed an increase in total length during this period but this difference was not significant (P > 0.05).In juveniles, specific activity of trypsin was significantly higher (P < 0.05) in the swimming fish than the bottom settled in the high density group. Trypsin level was significantly lower (P < 0.05) after two days of being transferred to lower densities. Swimming fish reared at high density presented the highest cortisol levels. Bottom settled fish in the high density group had higher levels than those in the lower density. On swimming fish transferred from high density to low density, cortisol levels tended to decrease after 4 days.These results showed that the rearing density has a significant influence on Japanese flounder juveniles, and results in social hierarchy (bottom settled > swimming) at high densities, that further causes higher cortisol levels and lower growth rates in low rank individuals.