Open-ocean foraging ecology of southern bluefin tuna Thunnus maccoyii based on stomach contents

Abstract

We investigated the foraging ecology of southern bluefin tuna Thunnus maccoyii in open-ocean habitats of temperate waters in the southern hemisphere by analyzing their stomach contents. Samples were collected from longline vessels over 15 yr (n = 4649). Of the prey, 51% by weight were cephalopods and 46% were teleosts. These values differ from those in the literature for other top predators in the open oceans, for which teleosts compose the largest portion of prey. The dominance of cephalopods also differs from the pattern for juveniles in previous studies in their coastal habitat, where most of the prey are teleosts. Thus, a distinct shift of prey occurs along with the habitat shift due to ontogenetic development. By weight, important prey were ommastrephid (18%), lycoteuthid (12%), and argonautid (1%) cephalopods and nomeid (8%, mainly Cubiceps caeruleus), paralepidid (7%), bramid (6%), and alepisaurid (6%) teleosts. The prey composition was relatively consistent among tuna sizes, sea surface temperatures, and years; changes in prey composition were due largely to differences in the cephalopod prey. Cephalopods belonging to the families Lycoteuthidae and Argonautidae contributed to the prey only off the southern coast of Africa and near Tasmania, respectively. Lycoteuthids occurred at lower sea surface temperatures than ommastrephids off the southern coast of Africa. Small ommastrephids were dominant in smaller tuna in the southeastern Indian Ocean. Our data provide basic information that will improve our understanding of the oceanic food webs in southern temperate waters.

Spawning area and season of butterfly kingfish (Gasterochisma melampus), a large scombrid adapted to cooler temperate southern water

“…There have been few studies of the ecosystems in the circumpolar temperate waters of the Southern Hemisphere at latitudes between 30 and 508S extending across the Atlantic, Indian and Pacific oceans (Itoh and Sakai 2016). Representative large pelagic fish species distributed in this area are the southern bluefin tuna (Thunnus maccoyii), albacore (Thunnus alalunga), butterfly kingfish (Gasterochisma melampus), slender tuna (Allothunnus fallai), opah (Lampris guttatus), blue shark (Prionace glauca) and the porbeagle (Lamna nasus).…”