Small pelagic fish constitute 25–40% of the fisheries landings in Mexico. More than 70% of these landings, predominantly Pacific sardine (Sardinops caeruleus), are captured in the Gulf of California. Small pelagic fishes are a key component of the Gulf's ecosystem; they are eaten by seabirds, sea mammals, and other fishes. The sardine fishery within the Gulf has been showing signs of overfishing since the early 1990s. To contribute to the sustainable management of this fishery, we developed two statistical models that use oceanographic conditions and seabird breeding and feeding data to predict total fishery catch and catch per unit effort (CPUE) of Pacific sardine in the central Gulf. Total catch was predicted with an accuracy of 54% by a linear model incorporating the Southern Oscillation Index (SOI), the clutch size of Heermann's Gulls (Larus heermanni), and the proportion of sardine mass in the diet of Elegant Terns (Sterna elegans). CPUE was predicted with an accuracy of 73% by a model based on the proportion of sardines in the diet of Elegant Terns, the reproductive success of Heermann's Gulls, and the springtime sea surface temperature anomaly in the Gulf region. Our results show that the reproductive ecology of seabirds is coupled to the global and local oceanographic conditions and that this information can be used to predict in advance the outcome of fishing efforts. We propose the use of models of this kind to reduce the effort of the fleet in years when it can be anticipated that CPUE will be low.
Small pelagic fisheries show wide fluctuations, generally attributed to oceanographic anomalies. Most data on these fisheries come from landings, often reporting sustained catches-per-unit-effort (CPUEs) until a decline occurs. Fishery-independent data are important as management tools. In this study we show that the proportions of Pacific Sardine and Northern Anchovy in the diet of three seabird species (California Brown Pelicans, Heermann's Gulls, and Elegant Terns) nesting in spring in the Gulf of California show significant relationships with CPUEs during the following season in gulls and terns, or during the same season in pelicans. As sardine availability for seabirds declines, CPUEs remain high until the fishery falls, one or two seasons later. A declining proportion of sardines in the seabirds' diet, combined with the status of the Pacific warm-phase anomaly (El Niño), give a reliable forecast of diminishing CPUEs and signals the need to reduce fishing efforts in the ensuing season.
Climate change and other human activities are causing profound effects on marine ecosystem productivity. We show that the breeding success of seabirds is tracking hemispheric differences in ocean warming and human impacts, with the strongest effects on fish-eating, surface-foraging species in the north. Hemispheric asymmetry suggests the need for ocean management at hemispheric scales. For the north, tactical, climate-based recovery plans for forage fish resources are needed to recover seabird breeding productivity. In the south, lower-magnitude change in seabird productivity presents opportunities for strategic management approaches such as large marine protected areas to sustain food webs and maintain predator productivity. Global monitoring of seabird productivity enables the detection of ecosystem change in remote regions and contributes to our understanding of marine climate impacts on ecosystems.
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