Exploring the ecosystem structure of Zanzibar archipelago, trophic flows and fisheries interaction by using a mass balance modelling approach

Abstract

The marine ecosystem in the Zanzibar archipelago has great economic and ecological importance due to high fisheries productivity. Yet, limited information is available on its status and possible impacts of fishing on it. Ecological models have been recognized as powerful tools for assessing the structure and function of ecosystems and forecasting the impacts of human activities on marine ecosystems. However, the use of ecological models in understanding ecosystems in the Western Indian Ocean region is inadequate. This study constructed a model to examine the ecosystem functioning, trophic flows, and fisheries interactions of the marine region affected by fishing gears in the Zanzibar archipelago using Ecopath with Ecosim software. The model includes 34 functional groups and 10 most common fishing gears, the input parameters were gathered from published reports. The ecosystem indicators showed that the Zanzibar ecosystem is in the early developmental stage contributed by strong fishing pressure that reduces the stability and maturity of the ecosystem. The total catch estimated at 5.538 t km(-2) year(-1) showed a sign of the high exploitation rate of fisheries resources. The catch is dominated by higher trophic level fish as reflected in the mean trophic level of the fishery (3.2). The keystoneness analysis identified pelagic cephalopods, sharks and rays, turtles, tuna and tuna like species and toothed whales for their importance in the Zanzibar marine ecosystem. Fishing pressure on the ecosystem of Zanzibar is high, hence for better management, the focus should be for coral reef and pelagic species that are highly targeted by number of gears while protecting keystone species as important groups of the ecosystem. Fisheries harvest data collection system should be improved to collect information at species level and focus on all target species, including invertebrates.