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Fisheries assessment of Chwaka Bay (Zanzibar) – following a holistic approach

Author:
Rehren, J., Wolff, M., Jiddawi, N.
Source:
Journal of applied ichthyology 2018 v.34 no.1 pp. 117-128
ISSN:
0175-8659
Subject:
Lethrinus, Siganus, boats, data collection, fisheries, fishermen, gears, juveniles, mortality, stocking rate, Zanzibar
Abstract:
This study investigates the typical East African multispecies fishery of Chwaka Bay (Zanzibar) by assessing growth, mortality, exploitation and stock size of six key target species of the fishery. Analyses are based on monthly length–frequency data collected from January to June and September to December in 2014 from the three main landing sites surrounding the bay. Estimated von Bertalanffy growth parameters using the ELEFAN I routine as implemented in FiSAT II varied among species from 27.6 to 64.4 cm (L∞) and 0.24 to 0.8 per year (K). Current exploitation rates compared to biological reference points calculated from yield‐per‐recruit analysis for five of the six key species indicate that Siganus sutor, Lethrinus borbonicus and Lethrinus lentjan are harvested beyond maximum sustainable levels (Eₘₐₓ). While juvenile retention rates of these three species are >80% and current Lc‐values are lower than Lₒₚₜ, fishing mortalities for Siganus sutor and Lethrinus borbonicus are highest for specimens above length at first maturity. Two management measures are discussed: (i) an increase in mesh size; and (ii) closure of the destructive dragnet fishery. The first option seems feasible only if the radius of the fishery were increased to capture larger specimens outside the shallow bay area. The second option would leave approximately 550 fishermen unemployed. Reallocating dragnet fishing effort to other gears would lead to a substantial increase in the number of boats, which might create use conflicts over the limited fishing area. The general status of the fishery in Chwaka Bay is considered as representing a ‘full exploitation to over‐exploitation scenario’, with no scope for expansion.
Agid:
5884011