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Title: Assessment of the artisanal fishery for Octopus cyanea Gray 1849 in Tanzania : catch dynamics, fisheries biology, socio-economics and implications for management
Author: Guard, Martin
ISNI:       0000 0001 3521 6715
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2003
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Catch dynamics, fisheries biology and socio-economic dependence are described for the artisanal fishery for Octopus cyanea at three study sites (Kwale. Jibondo, and Mtwara) along the coat 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 Iibondo 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 Jibondo. Abundance of octopus was however, higher at the former sites most likely in response to predator and competition release. Stock size and biomass (kg) were 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 tban 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 overtished and Kwale somewhere in between. Although, Bhattacharya modal progression analyses separated multiple size modes in monthly samples growth analyses were unsuccessful due to the 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 Msangambru 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 fisbed zone. Nonetheless, enough females would have to reach brooding size at each site to contribute to the reproductive stock. Size at first maturity for females was estimated to be 1800g. Only 3.9% of the octopus at Kwale and 1.2% at Msangamkuu weigh more than 1800g compared to 17% at Jibondo. Fecundity was shown to increase with size. Breeding activity is likely to be year round but maturity peaks were evident for June and July and October to November. The den enrichment experiment indicated a clear preference for concrete dens over PVC pipe. Overall density increased by 10% subsequent to den placement but dens were shown in reef areas not to be limiting. The benefit of den enrichment was therefore ambiguous but could be initiated using a phased approach to gauge effectiveness. The artisanal octopus fisheries make an important contnbution to the local economy. population growth and fisher and trader immigrations were noted as the greatest threats to the octopus fishery by fishers. Awareness of relevant marine resource use issues was shown to be high yet enthusiasm for management varied between sites. The first descriptive details of the octopus fauna of Tanzania are provided. Implications for management of the octopus fishery and a series of proposed management actions are provided for discussion between relevant stakeholders.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Octopus fishing