Print PageSeabirds, inhabit a highly variable environment, are long-lived, have high adult survival, low fecundity and deferred sexual maturity. World atmosphere and oceans exhibit co-variations over several temporal scales. At the interannual scale, El Niño-Southern Oscillation (ENSO) affects the Pacific Ocean large marine ecosystems, involving changes in atmospheric pressure, sea surface temperature and marine productivity. Seabirds are the most conspicuous of all marine organisms and upper-trophic level species; they “amplify” changes in oceanographic and mid-trophic level species attributes, such as small pelagic fish on which they feed. Consequently, they provide early signals of ecosystem change. Small pelagic fisheries often show wide fluctuations due to ENSO conditions. Fishery-independent data are necessary in the management of fisheries, and in estimating impacts of fishing on other ecosystem components. We consider oceanographic, climatic and ecosystem changes by examining three seabird species in the Gulf of California, one of the most productive marine ecosystems on earth. We analyze how population parameters, diet composition and fisheries catches have changed over time and covaried in relation to the Southern Oscillation Index (SOI). We examined Heermann’s Gulls demographic parameters, under ENSO and non-ENSO conditions, established models to analyze population growth under both situations, and performed simulations under different ENSO frequencies.
Results/Conclusions
One model predicted population growth during normal years, and rapid decline under ENSO. Under non-ENSO conditions fecundities contribute more to population growth than survival, and inversely under ENSO conditions. A simulation under different ENSO frequencies showed gradual decline of population growth as frequency increased. Heermann’s Gulls population can withstand up to one ENSO every 4 years, but will decrease drastically at higher frequencies. Seabirds longevity seems an evolutionary response to fluctuating environments. Heermann’s Gulls population can resist relatively high ENSO frequencies without compromising population growth, but may decline rapidly if warm-phase anomalies increase in frequency. Proportion of Pacific sardine and northern anchovy in the diet of three seabird species (California Brown Pelicans, Heermann´s Gulls and Elegant Terns) show strong non-linear relationships with sardine fishing fleet Catch-Per-Unit-Effort (CPUE). As sardine availability for seabirds declines, CPUE for the fishing fleet remains high. If harvesting is maintained, the fishery collapses. A declining proportion of sardines in the seabirds´ diet while CPUE remain high gives early warning that the stock is diminishing and fishing efforts should be restricted. The use of seabird diets as fishery-independent data can help monitor sardine stocks and prevent fishery collapses.
