Activity of digestive enzymes in yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus): indication of readiness for first feeding

The problem of determining when larvae should be offered food is particular ly difficult in a species such as Atlantic halibut that has a long yolk-sac period (280-320 degree days, dd). In order to help determine at what age A tlantic halibut larvae are able to digest food, we compared the activities of key digestive enzymes in four yolk-sac stages at an age interval that ha s been recommended for initiation of feeding, i.e., 161-276 dd. We tested t he hypothesis that digestive enzyme activities reach highest levels near th e end of this age interval. Activities of trypsin, amylase, lipase and alka line phosphatase were determined spectrophotometrically in whole yolk-sac l arvae at 161, 179, 230, and 276 dd. The activities of the same enzymes were measured in metamorphic larvae (660 dd) and in their Artemia prey to provi de reference levels from a fully developed digestive system and to estimate the importance of exogenous enzymes for Atlantic halibut larvae. Our resul ts showed significant (P < 0.001) differences in activities of all four dig estive enzymes among the yolk-sac stages with a general pattern of increase from 161 to 276 dd. Trypsin activities reached their highest values at 230 dd, whereas those of lipase and alkaline phosphatase peaked at 276 dd. Amy lase activities were detected only in the 230 and 276 dd stages, at statist ically indistinguishable levels. Based on percentage comparisons, specific activities of trypsin and amylase in whole 276-dd larvae were only 12% and 2%, respectively, of those measured in the digestive system of metamorphic larvae. whereas specific activities of lipase and alkaline phosphatase in 2 76-dd larvae were more than 50% of those determined for metamorphic larvae. The calculated contribution of enzyme activities derived from Artemia prey to the relatively high levels of enzyme activity in the digestive system o f metamorphic larvae was less than 10% for all enzymes except amylase, for which the contribution was estimated to be more than 50%. The results of th is study support our hypothesis that the highest digestive enzyme activitie s in volk-sac larvae are reached by 230-276 dd, i.e., near the end of the a ge interval recommended for first feeding. The observed pattern of enzyme a ctivities suggests that feeding of Atlantic halibut larvae should be initia ted after 230 dd. but not later than 276 dd to avoid the threat of starvati on. (C) 2000 Elsevier Science B.V. All rights reserved.