Intrapopulation structure of winter-spawned Argentine shortfin squid, Illex argentinus (Cephalopoda, Ommastrephidae), during its feeding period over the Patagonian Shelf

Stock structure dynamics of the important commercial squid Illex argentinus were studied by using bio-logical data from about 25 thousand squid caught January–April 1991 by Russian trawlers in three fishery regions: 51–52°S; 47–49°S within the exclusive economic zone of Argentina (EEZA); and 45–47°S outside the EEZA. A total of 2664 statoliths were read to prepare age-length keys for each 10-day interval of the period studied. It was found that between January and April, the Patago-nian shelf south of 45°S was a feeding ground of two intraspecific groups of winter-hatched I. argentinus: a shelf group that matured at medium sizes (ShG) and a slope group that matured at large sizes (SlG). After massive immi-gration of I. argentinus from the north in January–February into the two fish-ery regions within 45–49°S, the stock structure remained rather stable until April, composed predominantly of June- and July-hatched squid. Squid grow and mature rapidly, and males mature at younger ages (from one to two months) than do females. During feeding, some redistribution of the stock was observed: maturing and mature SlG squid (mainly females) tended to shift from the shelf (130–150 m depth) in a northeast direc-tion and concentrate over the shelf edge (160–170 m depth). In April, mature SlG squid began to shift to the continental slope around 45–47°S and migrated to depths >600 m where they then mixed with schools of SlG squid that had fed in the region 51–52°S and that were already migrating northwards along the slope. ShG squid remained on the shelf and made their prespawning northward migrations along the shelf edge