We present the ecological module of the BiOeconomic mArine Trophic Size-spectrum (BOATS) model, which takes an Earth-system approach to modeling upper trophic level biomass at the global scale. BOATS employs fundamental ecological principles and takes a simple approach that relies on fewer parameters compared to similar modelling efforts. As such, it enables the exploration of the linkages between ocean biogeochemistry, climate, upper trophic levels, and fisheries at the global scale.