Assessment of the artisanal fishery for Octopus cyanea Gray 1849 in Tanzania : catch dynamics, fisheries biology, socio-economics and implications for management
Catch dynamics, fisheries biology and socio-economic dependence is described for the artisanal fishery for Octopus cyanea at three study sites (Kwale, Jibondo, and Mtwara) along the coast of Tanzania. A total of 23,165 octopuses were measured and sexed from 3,514 individual catches. Fishing effort was shown to be 2.9 times greater at Msangamkuu compared to Jibondo and nearly two times more than Kwale. Number of relaxation days between fishing events, when the octopus stock are able to recover through growth and immigration, was shown to be greater at Jibondo than for the other two sites. Marked reductions in catch landings, mean individual weight, catch per unit effort (kg) and size distributions are reported for Msangamkuu and Kwale when compared to Jibondp. Abundance of octopus was however, higher at the former sites most likely in response to predator and competition release. Stock size and biomass (kg) was calculated for Msangamkuu using a De-Lury depletion method and results extrapolated to the other two sites. Despite higher abundance at Msangamkuu and Kwale mean overall biomass (kg) was 36% and 14% respectively lower than the mean biomass estimate for Jibondo. Spatial comparison of fishing impacts between sites using a surplus production model suggested Jibondo to be more productive and fishing pressure sustainable. In contrast, Msangamkuu was indicated to be seriously overfished and Kwale somewhere in between. Although, Bhattacharya modal progression analyses separated multiple size modes in monthly samples growth analyses were unsuccessful due to ttie lack of a clear corresponding pattern of modal progression in monthly size distributions. Length weight relationships varied between sites and sexes. All length weight relationships were negative allometric. Size reductions at Kwale and Msangamkuu may be impacting on reproductive output with a 40% reduction in the number of mature males at Msangamkuu and 24% reduction at Kwale when compared to Jibondo. Fewer mature females were observed at Msangamkuu and Kwale but even at Jibondo mature females accounted for only 2.2% of the catch. Results suggest spawning activity takes place in deeper water (>4m) below the fished zone.