The response of digestive enzyme activities and gut histology in yellowtail kingfish (Seriola lalandi) to dietry fish oil substitution at different temperatures

Yellowtail kingfish (Seriola lalandi) were fed five diets in which the dietary lipid component was replaced with 100% lipid as either poultry oil (PO), canola oil (CO), a blend of fish oil and poultry oil (FO/PO; 50:50) or a blend of fish oil and canola oil (FO/CO; 50:50) and held at 18 or 22°C. After five weeks, the changes in gastrointestinal histology and digestive enzyme activity of fish were examined. Digestive trypsin, lipase and α-amylase enzyme activities were downregulated in fish held at 18°C. The α-amylase activity was not influenced by diet, but the trypsin activities were significantly lower in fish fed the CO diet than fish fed the FO and FO/PO diets. Although the lipase activities were significantly lower in fish fed the CO diet than fish fed the FO/PO diet at 18°C and 22°C, there were no significant differences between fish fed other diets at both temperatures. The reduction in trypsin activity in fish fed the CO diet may be attributed to anti-nutritional factors present in the canola oil. The histology of the foregut and hindgut showed no signs of diet-induced enteritis. However, there was a high influx of goblet cells and severe reduction in supranuclear vacuolisation across all dietary treatments, including the fish oil control diet at both water temperatures and in initial fish samples. As no information exists on the histology of juvenile yellowtail kingfish this study provides baseline information for further study. However, reference tissue from wild fish is recommended to determine the validity of these findings and for the future histological assessment of this economically important fish species to dietary or environmental temperature changes.