A preliminary study of digestive enzyme activities and amino acid composition of early juvenile yellowfin tuna (Thunnus albacares)

The yellowfin tuna (YFT, Thunnus albacares) is a circumtropical/subtropical marine species that supports major fisheries throughout its range. Tuna ranching – an activity that currently relies on fish captured from the wild – has rapidly expanded, thus placing even greater pressure on wild tuna stocks worldwide. Although captive propagation of tunas has proven feasible, hatchery technologies for YFT need improvement to increase larval survival. The onset of exogenous feeding is a critical phase during which digestive processes develop rapidly. Therefore, the present research was undertaken to examine size-dependent changes in YFT fertilized eggs and in 23, 25 and 34 mm (total length) early juveniles with respect to the development of digestive capacity, biochemical composition and amino acid (AA) profiles as indicators of larval nutritional requirements for AAs. To this end, the activities of alkaline and acid phosphatases, pepsin, trypsin, chymotrypsin, aminopeptidase, lipase and α-amylase were determined spectrophotometrically. Activities of the analyzed enzymes were greater in the YFT than in similar-age/size marine fish of other species. In YFT, enzyme activities progressively increased as fish grew, resulting in significantly different (P < 0.0001) enzyme profiles among size classes. With the exception of acid phosphatase, all enzymes displayed a precocious increase in specific activity concurrent with somatic growth. Proximate composition and AA patterns were generally similar within juveniles but distinctly different (P ≤ 0.001–0.05) from that of eggs. Histidine, taurine and alanine were the most prevalent AAs in the free pool; whereas, arginine's free pool concentration increased substantially from egg to juvenile stages (2.2 to 12.7 nmol/mg, respectively). These results support the hypothesis of precocious digestive capacity for scombrid fish. Observed AA patterns suggest that strategies for feeding larval and juvenile YFT in aquaculture should incorporate higher levels of specific AAs during early development to support rapid growth and diminish the threat of starvation or cannibalism. Although this information is useful for understanding feeding processes in juvenile YFT, more research is necessary to fully characterize these processes in the various stages from egg to juvenile, leading to the development of balanced diets that would allow for early weaning from live prey.