Trophic Reorganization and Energy Deficit: A Multispecies Size-Spectrum Model of the Grand Banks

Marine ecosystems face unprecedented pressures from fishing and climate change, with both bottom-up energy transfer and top-down predation influencing ecosystem control, though their relative importance varies across space and time. The Grand Banks of Newfoundland provides a compelling case study, where capelin (Mallotus villosus) biomass collapsed by 99% in 1990–1991, followed by the collapse of Atlantic cod (Gadus morhua) and most groundfish species. Despite these dramatic shifts, the relative contributions of bottom-up versus top-down control mechanisms to ecosystem structure remain poorly understood. Here, we address this knowledge gap by developing a multispecies size-spectrum model of the Grand Banks fish community to simulate populations and community responses to scenarios of varying capelin, sand lance (Ammodytes dubius), and cod biomass recovery and depletion. Our results revealed contrasting patterns of ecosystem control depending on the scale of analysis. At the population level, cod removal generated biomass responses three times greater than equivalent forage fish reductions, indicating strong top-down control. However, these effects were largely buffered at the community scale through compensatory dynamics among non-target species. Conversely, capelin and sand lance depletion produced more modest population-level biomass changes but drove substantial biomass decline at community level. Among forage fish, capelin showed a more important role as energy source for the community. These findings suggest that Atlantic cod and capelin influence the Grand Banks through different mechanisms: trophic reorganization and energy deficit. Cod primarily structures the distribution of biomass among trophic levels, while forage fish govern total system biomass and energy. This work supports an ecosystem-based approach to management by providing a mechanistic understanding of how population changes in keystone predators and forage species lead to fundamentally different consequences for overall ecosystem structure and productivity.