Modelling the Multispecies Fishery of Chwaka Bay, Zanzibar - Basis for Exploration of Use and Conservation Scenarios

Abstract

Small-scale fisheries are essential livelihood and protein providers for coastal communities in developing countries. On the semi-autonomous island state Zanzibar (Tanzania) and, in particular, in Chwaka Bay, located on the east coast of the island, the dependency on small-scale fisheries is very high. Perceived decreases in individual catches and the use of destructive gears and small mesh sizes has led to concerns for an unsustainable use of the baya s fisheries resources. Particularly, the increasing use of the destructive dragnets has led to resource concerns and to strong conflicts between fishermen. However, the lack of knowledge on the state and dynamic of the fishery strongly impedes the potential for the development of proper management plans. The aim of the present dissertation is, therefore, to assess the status of Chwaka Baya s ecosystem and its fishery, as well as to evaluate potential use scenarios for a sustainable fisheries management. Furthermore, using Chwaka Bay as a reference site, the dissertation aims at approaching the answer to the question of the sustainability of Zanzibara s nearshore fisheries. The approach used in this dissertation is twofold: 1) length-based stock assessments were conducted for six of the main target species; and 2) a trophic flow model of the bay was constructed using Ecopath with Ecosim/Ecospace. The data used in both approaches were collected through an extensive field survey conducted over an annual cycle in 2014. A review of the literature about the state of Zanzibara s fisheries reveals that no fisheries assessments have been conducted after 2000. Analyses of the annual reported landings between 1990 and 2014 suggest that, except for clupeioids, none of the target groups of the fishery can be classified as overfished. Most studies aimed at evaluating the status of Zanzibara s resources have been focusing on ecological surveys and fishermena s perception. None of the ecological surveys appropriately link fishing effort and/or fishing pattern with resource state nor provide tangible thresholds for management. The stock assessment of the key target species of Chwaka Bay suggests that the exploitation rate of three out of the six target species exceeds recommended levels (E0.1). Despite high juvenile retention rates and length at first capture being below optimum length at first capture, fishing mortalities are highest for adults. Due to the nursery characteristics and the topography of the bay, juveniles might occur in higher abundances and larger fish may concentrate further outside the bay area. Consequently, an increase in mesh size only seems economically viable, if the radius of the fishery was increased to capture larger specimens outside the shallow bay area. The trophic model indicates that the Chwaka Bay ecosystem is comparatively mature. The bay is strongly bottom-up driven, with biomass concentrations around the first and second trophic level and a low overall fish biomass. The strongest impact on the ecosystem is exerted by dragnets and traps. Both gears potentially destabilize the ecosystem by reducing the biomass of top-down controlling species. Together with handlines, dragnets and traps are the least selective fishing methods. In addition, traps exert the highest fishing pressure on 4 out of the 6 selected key species. While the dragnet fishery is the least profitable, it also provides the highest number of jobs for the community, as it is a labor-intensive fishing method. Simulations of different use scenarios suggest that the elimination of dragnets would lead to the highest increase in overall fish biomass and profits. Nevertheless, this scenario would leave 58 % of fishermen without job and is, therefore, not feasible under the current lack of alternative livelihoods and the high dependency on fisheries resources. The complete reallocation of dragnets is likewise not feasible, since current effort is already high, and a further increase will lead to strong biomass reductions of target species and losses in individual profits of fishermen. Without compromising individual profits (-20 %) and biomass structure of the ecosystem (-30 %), only a part of the dragnet fishermen can be reallocated, leaving 37 % of fisher without jobs. In conclusion, the fishery of Chwaka Bay is fully exploited with some groups experiencing overfishing and does not provide scope for further expansion. Emperor fish together with similar vulnerable target resources such as Serranidae might be unsustainably harvested throughout the island. The lack of recognition for the capacity of dragnet boats to absorb surplus labour and their marginalization is likely hindering the development of feasible management plans aimed at regulating their use. In order to stop the use of dragnets on Zanzibar, management should focus on an effort control of this gear, while investing in the diversification of livelihoods.