Effects of salinity stress and diurnal variation on digestive enzyme activity of juvenile grey mullet Mugil cephalus

We studied the effects of salinity stress and diurnal variation on the protease and amylase activity of juvenile grey mullet (Mugil cephalus). Five different salinities were set: S0 (salinity is 0), S10, S20, S33 (control) and S40, respectively. The results indicate: 1) salinity stress has significant effects on activity of digestive enzymes (P0.05). Activity of protease and amylase increased within 0~7 d, then decreased within 7~14 d. At the last day (14th day), protease activity of S0~S33 increased while that of S33~S40 decreased. The difference of amylase activity of S0~S20 is insignificant (P0.05), andamylase activityof S20~S33 increased while that of S33~S40 decreased significantly (P0.05); 2) The diurnal variation in activity of digestive enzyme of M.cephalus reveals that the protease and amylase activity reached the maximum at 12:00 and 15:00, respectively, while bottomed at 6:00. Thus, feeding periodically at night can help the fish grow fast and healthily.