Oceanographic influences on squid population variability : Martialia hyadesi in the western South Atlantic
It has long been recognised that the life cycles of the major exploited ommastrephid squid species are closely related to oceanographic features, and that environmental variability may provide a significant stimulus for variability in the species abundance and distribution. In this thesis, the relationship between oceanographic processes and population variability for the ommastrephid squid Martialia hyadesi in the western South Atlantic is investigated using a geographic information system (GIS) (ARC/INFO(c) v. 7.2.1., ESRI Inc. 1999). From this analysis, it was clear that the oceanographic environment of the region does influence the abundance and distribution of M. hyadesi. However, the precise mechanisms by which this occurs are not determined. In the west of the study region, near the Patagonian Shelf, it appears that the distribution of M. hyadesi is intimately linked to that of the Falkland (Malvinas) Current, and that variability in the behaviour of this current may influence the annual abundance of the squid in the Patagonian Shelf squid fisheries. In the waters around the Falkland Islands (Malvinas), it appears that the 'population' of M. hyadesi may exist in two alternate states depending on its abundance. In most years, the squid occur at low densities, widely dispersed both in time and space. In exceptional years, the squid are far more abundant and are both spatially and temporarily aggregated. Although, no small juvenile or paralarval specimens of M. hyadesi were collected during this study, such specimens were collected for other squid species, and their distribution was successfully related to environmental factors. Both water mass type and water depth influenced the number of squid caught, and appeared to influence the species composition of the catches. Evidence was also found supporting the hypothesis that M. hyadesi does not occur near South Georgia during the austral summer and is extremely unlikely to spawn there.