Seasonal variations in pepsin, trypsin and chymotrypsin activities and protein digestibility were studied in yellowtails (Seriola quinqueradiata) reared for 1 year with extruded diets containing different levels of protein. Trypsin and chymotrypsin storage levels in the digestive tissues of starved fish were affected by seasonal changes in water temperature. Actual digestion activities of trypsin and chymotrypsin were low at lower water temperatures, but pepsin activity in the stomach tissue was not affected by low temperatures. On the other hand, trypsin and chymotrypsin activities in the intestinal contents were higher during lower water temperature months, while pepsin activity in the stomach contents was low at lower water temperatures. Apparent protein digestibility (APD) did not differ among the dietary treatments in the higher water temperature months, while in colder months it was higher in fish fed diet 1 than in fish fed diets 2 and 3. The APD values reflected pepsin activity in the stomach contents in all sampling months. Therefore, lower APD in colder months seems to be attributed to lower protease activity in the gastric digesta, implying that enhanced pepsin secretion from the stomach tissue might improve protein digestibility and growth performance in yellowtails during winter.

Feeding trials were carried out to examine substances that could be used as supplements in low-protein/ high-lipid diets to improve protein digestion of yellowtails (Seriola quinqueradiata) during winter. Dietary supplementation with synthetic or natural (krill and squid extracts) feeding stimulants (FS) improved feed intake and growth performance of yellowtails. Moreover, the apparent protein digestibility (APD) and pepsin, trypsin and chymotrypsin secretions in ¢sh fed diets supplemented with the di¡erent FS were superior to those in ¢sh fed the control diet. These ¢ndings suggest that the dietary inclusion of either synthetic or natural FS is a potential tool for improving protein digestion at low water temperatures, and may provide a promising solution for the winter growth retardation experienced in yellowtail culture.

The time required for trypsin restoration in the pyloric ceca-pancreatic tissue of yellowtail Seriola quinqueradiata (average body weight 942 g) was estimated. Fish were collected after starvation for 24, 48 or 72 h, and 6 h after being fed after each of the three tested starvation periods. Pyloric ceca isolated together with the pancreas were used for measurements of trypsin activity, quantity and expression rate. In the starved group, trypsin activities after 72 and 48 h starvation were significantly higher than after 24 h. Trypsin quantity, which was estimated by a Western blot test, showed similar results. The expression rate, estimated by reverse transcription polymerase chain reaction, was high after 24 h, but decreased after 48 h starvation. Thus, it was concluded that trypsin reaches the maximum storage level after 48 h starvation. In the fed group, the reduction in trypsin activity was greater when fish were fed after 48 and 72 h starvation periods than after 24 h. Trypsin expression rates 6 h after feeding were higher than before feeding. This indicates that trypsin synthesis is initiated after feeding or trypsin secretion and continues until 48 h of food deprivation. These results indicate that yellowtail of this size require feeding intervals of about 48 h to maximize trypsin storage.

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