Feeding habit of bluefin tuna Thunnus thynnus larvae in the western north Pacific Ocean

Abstract

This study examined the gut contents of 1, 939 larvae (2.28-14.60mm in total length) of bluefin tuna Thunnus thynnus collected in 1981 and 1982 by larva-net tows in the waters around Nansei Islands, and described their feeding condition and morphological development. The larvae fed on small zooplankton, mainly copepods, in the daytime: those less than 5mm TL on copepoda nauplii of less than 0.3mm in body length, and those larger than 5mm TL on larger copepods, especially of genus Corycaeus. This significant improvement of feeding ability at 5mm TL was also recognized both from examination of empty stomach rate and wet weight of gut content. The analysis of relative growth of the larvae showed that their external shape became constant at about 5mm TL. These results suggest that the larvae larger than 5mm TL fed more efficiently than smaller ones, getting higher chance of survival.