Population Structure of Martialia Hyadesi (Cephalopoda: Ommastrephidae) at the Antarctic Polar Front and the Patagonian Shelf, South Atlantic

Samples of the ommastrephid squid Martialia hyadesi were collected aboard Japanese squid fishing vessels during exploratory fishing operations at the Antarctic Polar Frontal Zone (APFZ) near South Georgia in February 1989 and during commercial fishing operations on the Patagonian Shelf in March 1986. Specimens caught during daylight at the APFZ were smaller than those caught during darkness. Specimens caught at the APFZ during darkness were generally smaller and less sexually mature than those from the Patagonian Shelf. Principal components analysis of character indices revealed no discernible separation between the sexes or between specimens from the two areas. Comparison of means of individual character indices for specimens from the two areas revealed differences between some areas for some characters, but in several cases the possibility that these differences were size related could not be discounted. Statoliths from samples taken from the modal mantle length at the APFZ were sectioned to reveal growth rings. If the rings are deposited daily, as in some other squid species, the age of these specimens is estimated to be 9–12 months. An unusually large bycatch of Martialia hyadesi in the Illex argentinus fishery on the Patagonian Shelf in 1986 was associated with low sea-surface temperatures in that year. It is proposed that the life cycle of Martialia hyadesi is associated with the West Wind Drift in the vicinity of the APFZ, that the spawning grounds lie to the west of the Scotia Sea and that the occurrence of the species on the Patagonian Shelf in 1986 was related to cold water conditions in the area which arose either through a northwards incursion of cold water or wind driven upwelling.