THE ROLE OF SLENDER TUNA, ALLOTHUNNUS-FALLAI, IN THE PELAGIC ECOSYSTEMS OF THE SOUTH-PACIFIC OCEAN

Gonad condition and stomach contents were examined in 2257 specimens o f Allothunnus fallai (463-896 mm fork length) caught by surface driftn ets from the high seas of the South Pacific (20-54-degrees-S, 74-150-d egrees-W) during 1985-1987. Subtropical waters and the Peru Current no rth of 31-degrees-S were considered to be the spawning grounds of slen der tuna during October, November and December. From November to Febru ary, feeding grounds were located in subantarctic waters and the Peru Current south of 38-degrees-S, where slender tuna preyed mainly upon e uphausiids, squids, copepods, young myctophid fish and amphipods. The dominant species in the diets were Euphausia vallentini, E. lucens, Th ysanoessa gregaria and Neocalanus tonsus throughout the subantarctic w aters. Young of the squid, Martialia hyadesi, dominated in the Southea st 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 impor tant in the diet. Adult slender tuna were preyed upon by Isurus oxyrin chus, Prionace glauca, Xiphias gladius and Makaira mazara in subtropic al waters, but not so in subantarctic waters. Allothunnus fallai was c onsidered to be a highly migratory species, with a life-style adapted to the seasonally fluctuating biomass of zooplankton in subantarctic e pipelagic waters.