The Role of Slender Tuna, Allothunnus fallai, in the Pelagic Ecosystems of the South Pacific Ocean

Abstract

Gonad condition and stomach contents were examined in 2257 specimens of Allothunnus fallai (463-896 mm fork length) caught by surface driftnets from the high seas of the South Pacific (20-54 o S, 74-150 o W) during 1985-1987. Subtropical waters and the Peru Current north of 31 o S were considered to be the spawning grounds of slender tuna during October, November and December. From November to February, feeding grounds were located in subantarctic waters and the Peru Current south of 38 o S, where slender tuna preyed mainly upon euphausiids, squids, copepods, young myctophid fish and amphipods. The dominant species in the diets were Euphausia vallentini, E. lucens, Thysanoessa gregaria and Neocalanus tonsus throughout the subantarctic waters. Young of the squid, Martialia hyadesi, dominated in the Southeast Pacific and Moroteuthis spp. were common in the subantarctic waters of the central South Pacific. Hyperiid amphipods, Primno macropa and Themisto gaudichaudii, were common but gravimetrically much less important in the diet. Adult slender tuna were preyed upon by Isurus oxyrinchus, Prionace glauca, Xiphias gladius and Makaira mazara in subtropical waters, but not so in subantarctic waters. Allothunnus fallai was considered to be a highly migratory species, with a life-style adapted to the seasonally fluctuating biomass of zooplankton in subantarctic epipelagic waters