Ocean Heat Content Variability and Its Influence on Primary Productivity and Fisheries Recruitment in the Bight of Benin

Ocean heat content variability drives cascading effects through marine food webs, from primary productivity to recruitment success of commercially important fish species. The mechanisms linking heat content anomalies to recruitment dynamics in the Bight of Benin are theoretically hypothesised but remain empirically poorly constrained. This research investigates the physical oceanographic controls on primary productivity variability and their propagation through the food web to fish recruitment in the Bight of Benin, using a combination of satellite remote sensing, in-situ oceanographic surveys, and a coupled physical-biogeochemical-fish recruitment model. Argo float data, NOAA sea surface temperature, and NASA MODIS chlorophyll-a products were analysed for the period 2000 to 2024. In-situ surveys were conducted in 2023 and 2024 aboard the Nigerian Institute for Oceanography and Marine Research research vessel. Acoustic surveys provided zooplankton biomass and mesopelagic fish density. Recruitment indices for five commercially important species were derived from port sampling programs and historical trawl surveys. A coupled physical-NPZD-larval fish bioenergetics model was developed in ROMS and validated against the observational dataset. Ocean heat content anomalies explained 58 percent of the interannual variability in thermocline depth, which in turn controlled nutrient upwelling intensity. Interannual variability in chlorophyll-a was significantly correlated with thermocline depth variability (r = negative 0.71). Lagged correlations confirmed that heat content anomalies predicted recruitment strength of Sardinella spp. with an 8 to 14 month lag. Model projections indicate a 22 to 38 percent reduction in mean chlorophyll-a by 2060 under high-emission scenarios, translating to a projected 18 to 29 percent reduction in small pelagic fish recruitment. Keywords: ocean heat content, primary productivity, fisheries recruitment, Bight of Benin, coupled modelling.