Abstract
AbstractWe hypothesized that change in the annual population size of guano‐producing seabirds (cormorant, Phalacrocorax bougainvillii; booby, Sula variegata; pelican, Pelecanus thagus) is a response to changes in primary and secondary production of the Peruvian upwelling system. We tested this hypothesis by modeling nitrate input through upwelling to the upper layers of the ocean off Peru between 6° and 14°S using data on wind stress and sea surface temperature. The model predicted the amount of carbon fixed by primary production each year from 1925 to 2000, which was then apportioned to the Peruvian anchovy (Engraulis ringens) biomass and ultimately to the seabird population and the anchovy fishery, the largest single‐species fishery on Earth. The model predicted a marked increase in primary production as a consequence of increasing wind stress. It overestimated the anchovy biomass after the collapse of the fishery in 1972, but closely predicted the growth of seabird populations from 1925 to the mid‐1960s, and their decline thereafter, explaining about 94% of the variation in seabird numbers from 1925 to 2000. The model indicates the seabirds consumed 14.4% of the available anchovies and, thus, that seabirds consumed 2.3% of the new production, before the development of the anchovy fishery, and only 2.2% of the available anchovies and 0.3% of the new production after the development of the fishery. The model results clarify the roles that environmental and anthropogenic factors may have had in regulating the guano‐producing seabird populations. It indicates that the growth of seabird populations from 1925 to 1955 was likely a response to increased productivity of the Peruvian upwelling system and that the subsequent drastic decline in seabird abundance was likely due to competition for food with the fishery, which caught ∼85% of the anchovies, which otherwise would have been available for the seabirds. This model also shows that an increase in oceanic primary production promotes reproductive success and population growth in higher trophic level organisms.
Highlights
Eastern boundary current systems of the world’s oceans are among the most productive marine environments, supporting rich and diverse communities (Cushing, 1971; Hill, 1998; Carr, 2002)
We investigated the relative importance of winddriven input of nutrients and the removal of Peruvian anchovies by the fishery to the population dynamics of guano-producing seabirds
The model indicates that primary production off the coast of Peru increased from 1925 to 2000 in response to increased wind stress observed in the modeled area
Summary
Eastern boundary current systems of the world’s oceans are among the most productive marine environments, supporting rich and diverse communities (Cushing, 1971; Hill, 1998; Carr, 2002). The guanay cormorant (Phalacrocorax bougainvillii), Peruvian booby (Sula variegata), and Peruvian pelican (Pelecanus thagus) are the most abundant seabirds in the Peruvian current region (Murphy, 1936; Jahncke, 1998) They inhabit the west coast of South America, within the cool upwelling zone from Isla Lobos de Tierra (06°8¢S) off northern Peru to Isla Mocha (38°30¢S) off southern Chile (Murphy, 1936). Guano was extracted initially for gunpowder production and later for fertilizer (Muck and Pauly, 1987) These seabirds feed primarily on Peruvian anchovies (Engraulis ringens), the most abundant pelagic fish of the Peruvian upwelling system (Jahncke and Goya, 1998 and references therein; Table 1). We predicted that seabird numbers should be positively related to anchovy pro-
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