Abstract:

The California Current hosts diverse and valuable fish communities shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes is poorly understood. Here, we investigate these interactions in a regional configuration of the APex ECOSystem Model (APECOSM) in the California Current. APECOSM is a mechanistic model of the pelagic food web that resolves predator-prey interactions, fish swimming, and transport by currents. Forced by an eddy-resolving representation of ocean currents and lower trophic levels, the model reproduces observed fish biomass from fisheries-independent trawls, and the observed succession from small to large sizes moving from the coast to the open ocean. We show that, in the model, the cross-shore variation emerges from a combination of (1) passive offshore advection by the mean surface current, (2) active swimming towards coastal productive regions, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. The model also helps explain the lower biomass and typically smaller size of surface-dwelling species compared to mid-water migrating species by the temperature dependence of metabolism. This work shows the first successful implementation of a regional size-structured fish model and highlights the importance of fish movement and currents in shaping the pelagic food web of the California Current.