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This study investigates the effects of gradual or abrupt changes in rearing salinity on food transit time and digestive enzymes activity of Caspian kutum (Rutilus kutum) larvae. The larvae (532 AE 0.05 mg) were supplied and randomly allocated into 12 tanks at a density of 45 fish per tank. Experimental treatments were fresh water (salinity 0) [FW] as control, exposure to salinity 5 [T 1 ], and gradual transfer to salinity 10 in two steps of first to 5 h, then and after 12 h to a salinity of 10 [T 2 ], and abrupt change (direct transfer to a salinity of 10 [T 3 ]). Results showed at 8 h after start of feeding that the larvae intestine was filled with food pellets except in treatment T 1 . Enzyme activity responded to salinity change as follows: the highest trypsin, amylase, and chymotrypsin activities were observed in T 1 ; however, these were not significantly different to treatment T 3 (P > 0.05). Trypsin activity peaks in the FW and T 2 groups occurred 8 h after feeding, and in T 3 and T 1 groups 5 h after feeding. Peak chymotrypsin and alkaline phosphatase activity was observed 5 and 8 h after feeding in all experiments, respectively. The highest a amylase activity in FW and T 2 groups occurred 5 h after feeding, while in T 3 and T 1 these peaks were observed 8 h after feeding. These results indicate that salinity had some noticeable effects on the activities of digestive enzymes after feeding.

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