A Study of Neotropic Cormorants in Central Chile: Possible Effects of El Nino

-In 1997, one of the strongest recorded El Niiio-Southern Oscillation (ENSO) events occurred in the eastern Pacific. It caused strong oceanic and climatic anomalies, such as high sea surface temperatures (SST) and heavy rainfall along the eastern Pacific coast. We investigated the effects of this event on the reproductive ecology of the Neotropic Cormorant (Phalacrocorax brasilianus) in central Chile (36037'S; 73'03'W) by comparing the preEl Nifio season in 1996 with the 1997 El Nifio year. Fish landings immediately prior to the 1997 breeding season were notably lower and rainfall increased. In 1997, the number of breeding pairs declined by 71% from the previous year. During El Nifio, laying started 15 days later, was less synchronized and ended 35 days later. Although egg size, clutch size and hatching success did not differ significantly between the two years, the decrease in the breeding numbers in 1997 would have caused a considerable drop in offspring production. We suggest that poor environmental conditions may have caused young or poor quality birds to miss breeding in the El Nifio year. Since it appears that large El Nifio events are becoming more frequent, it is important to undertake more research into the ability and mechanisms of seabirds to cope with them. Received 12 February 2001, accepted 26June 2001.