Landscape dynamics and resulting species interactions: the cod-capelin system in the southeastern Bering Sea

Capelin area major component of cod diet in many ecosystems of the North Atlantic. In the Bering Sea, however, the percentage of capelin found in Pacific cod stomachs is negligible. We hypothesize that the landscape (or 'seascape') features of hydrography and bathymetry of the Bering Sea impose a constraint on the distribution of cod and capelin that contributes to their spatial segregation and ultimately, to their uniquely weak trophic link. We analyzed a long-term data collection (1972 to 2001) on Pacific cod and capelin catches and feeding interactions, and developed a new metric to quantify the spatially explicit overlap between a predator and a prey. Our analysis indicates that cod and capelin distributions respond to the thermal conditions of the Bering Sea. Typically, a frigid water mass is present on the middle portion of the shelf, and cod distribution is confined to the southern side of the cold pool, while capelin is confined to the north. However, in warm years the middle shelf is penetrated by thermal gateways (between 1 and 6 degrees C), allowing cod to invade the capelin habitat. In this situation, when cod and capelin overlap, cod feed heavily on capelin. The annual index of cod and capelin overlap is correlated with the Pacific Decadal Oscillation (PDO) index, a climate indicator of the North Pacific. The width of the thermal gateways in the middle shelf is also correlated with the PDO. These results point to a direct causal link, from climate and thermal conditions of the Bering Sea shelf, to cod and capelin distribution, and ultimately to their overlap and trophic interactions.