Aspects of the population biology of Octopus vulgaris in False Bay, South Africa

Abstract

The population biology of the octopus Octopus vulgaris was studied from specimens collected by SCUBA in False Bay, South Africa, between 1997 and 1998. In all, 83% of the specimens collected were found in shelter. Small octopuses were more active than large individuals during the day, 37% of the former and 8% of the latter being found outside of shelters. All males >170 g were mature, the smallest male being 136 g. No mature females were found in the study area, and maturing females spanned a broad size range caught (275-3 600 g). Average size at spawning is probably between 1 500 and 2 500 g. Although there were no significant overall differences in mass between sexes (p > 0.05), the average mass of octopus in winter (766 g) was significantly smaller than in summer (1 161 g) and spring (964 g). Fluctuations in the average size of octopus tracked subsurface water temperature. The overall sex ratio was significantly biased towards males (0.6F:1.0M, p < 0.01), largely because fewer females were caught during spring and summer. Spawning likely occurs throughout the year. However, significantly higher (p < 0.01) female gonadal somatic indices of 0.52 and 0.46 found in spring and summer respectively (periods of warmer water in False Bay) may indicate peak spawning during those seasons.