The biomass and ecology of chokka squid Loligo vulgaris reynaudii off the West Coast of South Africa

Abstract

Migration, stock size and ecology of chokka squid Loligo vulgaris reynaudii off the West Coast of South Africa were studied and their relationship to other regions compared by analysis of distributional, biomass, and size composition, and biological data collected from biannual research cruises from 1983-1987. Biomass was lower than on the South Coast, higher in summer than in winter and declined over the duration of the study. The distribution of squid was generally continuous from the east up to between Cape Point and Cape Columbine. North of Cape Columbine it was more sporadic and occurred only occasionally in the far north (north of 31°S). Depth distribution, at up to 350 m, was much greater than on the S. Coast where an insignificant percentage of the biomass occurs over 200 m. The size composition was unimodal compared to a polymodal distribution on the S. Coast, and mean and modal sizes were smaller. Maturity rates and gonado-somatic indices were also much lower. The percentage of feeding squid was, however, considerably higher. These findings, together with distributional and commercial catch data, showed that cyclic immigration and emigration occurs between the two areas, probably to enable subadult squid to exploit the good foraging opportunities on the W. Coast. A multiple correlation analysis and regression model showed that catches were correlated with a combination of bottom depth, bottom temperatures and bottom oxygen levels. Bottom temperatures of below 8°C and bottom oxygen levels of below 3,5 ml^-1 appeared to represent important limiting factors in the distribution.