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 (P < 0.05).Activity of protease and amylase increased within 0~7 d, then decreased within 7~14 d. At the last day (14^th day), protease activity of S₀~S₃₃ increased while that of S₃₃~S₄₀ decreased. The difference of amylase activity of S₀~S₂₀ is insignificant (P>0.05), and amylase activity of S₂₀~S₃₃ increased while that of S₃₃~S₄₀ decreased significantly (P < 0.05); 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.