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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 <i>Martialia hyadesi</i> 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 <i>M. hyadesi</i>. 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 <i>M. hyadesi</i> 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 <i>M. hyadesi</i> 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 <i>M. hyadesi</i> 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 <i>M. hyadesi</i> does not occur near South Georgia during the austral summer and is extremely unlikely to spawn there.

The research involved development of two ecological simulation models to explain the complex dimensionality of chinook salmon (Oncorhynchus tshawytscha) life history structure (represented by the age composition of the spawning stock) and management difficulties entailed in the complexity. Since different sizes of chinook salmon are thought to adapt differently to heterogeneous habitats, age composition of the spawning stock is determined by characteristics of the habitats of the substocks. Numerical properties of substocks result from the incorporation of individual spawners in different age classes and each substock performs differently because their age compositions are distinctive. A stock or population consists of substocks whose age compositions are concordant with their habitats. The productive capacity of a population will result from the incorporation of substocks. If habitat structures of streams are different, the age and size compositions and productive capacity of the populations may differ. Selective harvesting affects spawners in different ways, so that age compositions must be deformed differently by fishing pressure. Once the age composition deviates from the natural age composition, the productivity of the population will decrease. Population dynamics are strongly correlated with substock structure which is determined by habitat structure in a stream system. Hierarchical population structure make fisheries management difficult and requires not only quantitative but also qualitative analysis on the populations in relation to habitat classification. / Graduation date: 1990

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