The biomass and ecology of chokka squidLoligo vulgaris reynaudiioff the West Coast of South Africa

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.1-1 appeared to represent important limiting factors in the distribution. Trekgewoontes, stapelgrootte en die ekologie van die tjokka Loligo vulgaris reynaudii aan die Weskus van Suid-Afrika is bestudeer en verwantskappe met ander gebiede vergelyk aan die hand van ontleding van verspreidings-, biomassa-, grootteverspreidings-en biolog ¡ese data wat twee maal jaarliks tussen 1983 en 1987 versamei is. Biomassa was laer as aan die Suidkus, hoër in die somer as gedurende die winter en biomassaskattings het tydens die studie gedaal. Die verspreiding van tjokka was oor die algemeen aaneenlopend vanaf die ooste tot tussen Kaappunt en Kaap Columbine. Noord van Kaap Columbine was dit meer sporadies, en het slegs af en toe in die verre noorde (noord van 31 °S) voorgekom. Diepteverspreiding was tot 350 m, heel wat dieper as aan die Suidkus waar 'n onbeduidende persentasie van die biomassa dieper as 200 m voorkom. Die grootteverspreiding was unimodaal vergeleke met 'n polimodale verspreiding aan die Suidkus, en gemiddelde en modale groottes was kleiner. Die mate van geslagsrypheid en gonado-somatiese indekse was ook baie laer. Voedingstempos was egter heelwat hoër. Hierdie bevindings, tesarne met verspreidings-en vangsdata, het daarop gedui dat daar 'n sikliese immigrasie en emigrasie tussen die twee gebiede plaasvind, waarskynlik sodat voorvolwasse pylinkvis die goeie voedingspotensiaal aan die Weskus kan benut, 'n Meerveranderlike korrelasie-analise en regressiemodel het bewys dat vangste gekorreleerd was met 'n kombinasie van bodemdiepte, bodemtemperatuur en bodemsuurstofvlakke. Bodemtemperatuur onder 8°C en bodemsuurstofvlakke onder 3,5 ml.l-1 is blykbaar belangrike fakore wat beperkend inwerk op verspreiding