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vol.30 número1  suppl.SympThe 1997/98 El Niño in the Humboldt Current System: a Synthesis1992 El Niño Effect over some Population Parameters of Northern Anchovy from the Gulf of California: A Comparison with Anchovies from the California and Humboldt Currents índice de autoresíndice de materiabúsqueda de artículos
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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Effects of El Niño 1997-98 on the Diet
Composition and Numbers of the
Peruvian Guano-Producing Seabirds

Elisa Goya, Aquiles Garcia-Godos

Instituto del Mar del Perú, Apartado Postal 22,
Callao Perú,

Food availability is one of the main factors in the regulation of seabird populations. In order to examine biological responses of Peruvian guano birds to fluctuations in prey abundance and distribution, we analyzed 29,773 pellets of Guanay Cormorants (Phalacrocorax bougainvillii) and 4,675 regurgitations of Peruvian Boobies (Sula variegata) over six consecutive years (1996-2001) at five - eight localities along the Peruvian coast. Additionally, we estimated numbers of adult guano-producing seabirds (Guanay Cormorants, Peruvian Boobies and Peruvian Pelicans) at protected islands and headlands in Peru between 1995 and 2001 in order to evaluate how El Niño 1997-98 affected the guano bird population.

In non-El NIño years, the main prey of Guanay Cormorant was the Peruvian Anchovy (Engraulis ringens), while the Peruvian Silverside (Odonthestes regia regia) was the alternative prey in southern Peru. Both prey accounted for almost 85% by number in the diet. Latitudinal diet variation matched the large-scale distribution pattern of Peruvian anchovy: high availability in the northern-central coast of Peru (4°-14°S), and low availability in the south (16o-18oS). Despite the fact that anchovy was the main prey of the Guanay Cormorant and the Peruvian Booby during the study period, there were differences in its relative abundance in the diet among years. The anchovy consumption of Peruvian Boobies during 1998 was the lowest (58%), whereas the Peruvian pilchard Sardinops sagax consumption increased. We compared annual fluctuations in the abundance of anchovy in Peruvian Booby's diet (percentage by mass) with commercial fishery landings (million tonnes) to evaluate whether prey harvest by seabirds and fishery are similarly influenced by local fluctuation in prey availability. There were no significant correlations between the variables (rSpearman =0,714, p>0.05, n=7), nevertheless, we observed a positive association. This association was particularly evident in 1998, when anchovy landings were low due to El Niño and the anchovy abundance in the Peruvian Booby's diet decreased significantly.

Before El Niño 1997-98 the guano bird populations were slowly increasing, with a peak of 6.8 million individuals in 1996. El Niño 1997-98 severely affected bird numbers in Peru as a consequence of dispersion from North-Central Peru to southern Peru and Chile, nest desertion and mortality of immature birds and adults. Late in 1997 the guano bird populations in Peru decreased by 99.4% for the guanay cormorant, 87.1% for the Peruvian Booby and 99.4% for the Peruvian pelican Pelecanus thagus. Currently, the guano bird population levels are critical (1.8 million individuals) and, during the last two years, no significant increase has been reported. Our data suggest that the adult reproductive rate has decreased in recent years. Many studies have shown that the adult reproductive rate and the reproductive success decrease during low food availability periods, because of a trade-off between lifetime reproductive success and actual adult survival.

The low anchovy availability during El Niño 1997-98 determined the mortality and dispersion of guano birds southward and the severe reduction of their populations in Peru. However, seabirds are also influenced by fishery-induced changes, and both natural and human perturbations have interactive influences on seabird feeding ecology and reproduction. Although the anchovy population has recovered after El Niño 1997-98, the commercial harvest may decrease the anchovy availability for the guano birds and would limit their population recovery. Likewise, a higher occurrence of El Niño events would negatively affect the adaptability of guano birds to global environmental changes.

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