Summer distribution, abundance and population structure of Illex argentinus on the Argentine shelf in relation to environmental features

Abstract

The distribution, abundance and population structure of Illex argentinus in relation to oceanographic conditions during summer 1996 were analysed following a research cruise to the Argentine shelf (39°–51°30′S). Squid were distributed over the whole area, but the main concentrations were on the intermediate and outer Patagonian shelf (44°30′–47°30′S), where there was a thermal front (12–15°C). Three squid groups were detected through study of the distributional area, size, maturity stage, age and hatching month. North of 41°S, there were immature individuals of 14–21 cm mantle length (ML) that had hatched mostly in June. Small mature, spawning and spent squid (males 14–22 cm ML, females 18–26 cm ML) were found on the inner and intermediate shelf between 40°30′ and 46°30′S. They had hatched from January to April, with a clear peak in March. South of 45°S, males of 17–25 cm ML and females of 17–29 cm ML were found over the intermediate and outer shelf; most males were mature whereas the majority of the females were immature. They had hatched from May to July, with a peak in June. The relationship between statolith increment and size showed that the growth rates of the later-hatched individuals were faster than those of individuals hatched earlier. The squid preyed almost exclusively on zooplankton (87%), mostly the hyperiid amphipod Themisto gaudichaudii (79%).