Mechanistic Drivers of Fish Stock Collapse in the Bight of Benin: An Ecosystem Dynamics Approach

The Bight of Benin pelagic fish stocks, historically among the most productive in West Africa, have exhibited pronounced declines in biomass and catch per unit effort over the past three decades, yet the relative contributions of overfishing, oceanographic variability, climate change, and habitat degradation to observed stock trajectories have not been rigorously disentangled. This dissertation applies an ecosystem dynamics approach to identify and quantify the mechanistic drivers of fish stock decline in the Bight of Benin, focusing on the Nigerian shelf sector. An extended time series of fish stock abundance indices is reconstructed from commercial catch statistics (1985 to 2024), scientific survey trawl data, and harmonised stock assessment outputs from the Fishery Committee for the Eastern Central Atlantic. A multispecies surplus production model with environmental covariates links stock biomass trajectories to time-varying oceanographic drivers extracted from ERA5, AVISO altimetry, and MODIS satellite products. An extended Ecopath with Ecosim ecosystem model of the Bight of Benin shelf is constructed incorporating 32 functional groups and three fishing fleet types, driven by the oceanographic forcing time series. Mixed fishing simulation scenarios test the sensitivity of ecosystem trajectory to alternative fishing mortality reduction policies. Bayesian state-space stock assessment models are applied to three commercially important species (Sardinella aurita, Pseudotolithus elongatus, Dentex angolensis) to partition observation error and process error in abundance trends and estimate probabilities of stock collapse under reference point frameworks. Path analysis identifies upwelling intensity decline (driven by weakening trade winds) and juvenile overfishing as the two most significant co-drivers of observed biomass decline. Keywords: fish stock collapse, Bight of Benin, ecosystem dynamics, Ecopath, stock assessment