Effects of photoperiod on growth, mortality and digestive enzymes in miiuy croaker larvae and juveniles

The growth, mortality and digestive enzymes (trypsin, amylase and lipase) in miiuy croaker Miichthys miiuy larvae and juveniles (253 dph) were investigated at four photoperiods: 24L:0D, 18L:6D, 12L:12D and 0L:24D. Larvae could not feed at 0L:24D and did not survive up to  dph. In the 24L:0D, 18L:6D, 12L:12D groups, photoperiod had not significant effects on the growth of the miiuy croaker younger than  dph. However, their total length and specific growth rate (SGR) were significantly larger at 18L:6D and 24L:0D than 12L:12D after  dph. Photoperiod also affected the mortality of the first feeding larvae ( dph), being apparently higher in 27%), but no significant differences in mortality were found among other photoperiods. High mortality of the miiuy croaker in 12L:12D, 18L:6D and 24 L:0D groups mainly occurred from 5 (2027%) to 16%) and tended to decrease gradually from  dph onwards. Digestive enzymes activities in the miiuy croaker larvae and juveniles had a similar change trend with age at all photoperiods. They underwent drastic changes with age. The specific activity of lipase was significantly higher at 18L:6D and 24L:0D than 12L:12D, but there were no significant differences in specific activities of either trypsin or amylase among photoperiods. With regard to the total length, SGR, survival and digestive enzyme activities, our findings suggested that the optimal light regime for the culture of miiuy croaker during the early life stage was 18L:6D.