An ecosystem-based hypothesis for climatic effects on surplus production in California sardine (Sardinops sagax) and environmentally dependent surplus production models

We used environmentally dependent surplus production (EDSP) models to test hypotheses linking changes in habitat area, carrying capacity and surplus production in California sardine (Sardinops sagax). Habitat area (with mean sea surface temperatures of 14-16 degrees C) was centered off Oregon, Washington, and British Columbia during July-December and off southern and central California during January-June. Habitat area increased during El Nino and decreased during La Nina events. EDSP models fit better than a conventional Fox surplus production model without environmental data. Our estimated fishing mortality rate at maximum sustained yield F-MSY = 0.099 center dot year(-1) was consistent with other estimates. Maximum sustained yield (MSY) and stock biomass for MSY (B-MSY) depend on habitat area and environmental conditions. Negative surplus production occurred when biomass was high and habitat area declined abruptly. Managers might monitor habitat area to anticipate changes in the California sardine stock and changes in the California Current ecosystem. Periods of high productivity appear easier to identify than periods of negative productivity. Models that incorporate environmental effects on both recruitment and survival and mortality of adult fish appear useful in studying climatic effects on fishery surplus production.