Trophic Structure and Energy Flow in the Coastal Pelagic Ecosystem of Bight of Benin Using Ecopath Modelling

Mass balance ecosystem models provide a quantitative framework for understanding trophic interactions and estimating the impacts of fishing on marine food web dynamics. This study constructed an Ecopath model for the coastal pelagic ecosystem of the Bight of Benin, incorporating biological and fisheries data from Nigerian waters. The model comprised 18 functional groups ranging from phytoplankton and zooplankton to apex predators and artisanal fisheries. Key biological parameters including production-to-biomass ratios, consumption-to-biomass ratios, diet compositions, and ecotrophic efficiencies were compiled from field surveys, stomach content analyses, and published literature. Ecosim dynamic simulations were applied to assess the ecosystem response to changes in fishing effort over a 25-year projection period. Phytoplankton production was estimated at 3.8 tonnes of carbon per square kilometre per year. Ecotrophic efficiencies exceeded 0.9 for most intermediate trophic level groups, indicating that almost all production was being utilised within the food web. Mixed trophic impact analysis showed that increases in small pelagic fishing effort generated negative cascading effects on piscivore biomass but positive responses in zooplankton groups due to trophic cascade release. Fishing in balance index values indicated that the current fishery is unsustainably extracting energy from the system. Simulation of a 30 percent effort reduction for small pelagics resulted in significant biomass recovery in upper trophic level species. The study provides the first mass balance model for this ecosystem and establishes a baseline for ecosystem-based fisheries management in the Bight of Benin. Keywords: Ecopath, trophic structure, Bight of Benin, ecosystem modelling, fisheries management.