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El Nino-Southern Oscillation (ENSO) events cause a decline in upwelling-based primary productivity throughout the California Current system and southern Gulf of California However, in the Canal de Ballenas, central Gulf of California, primary productivity is based on tidal mixing and appears unaffected by ENSO events. Between the ENSO year of 1983 and the anti-ENS0 year of 1985 we censused 4 piscivores (Bryde's whale Balaenoptera edeni; common dolphin Delphinus delphis; blue-footed booby Sula nebouxii and brown booby S. leucogaster) and 3 planktivores (fin whale B. physalus; black storm-petrel Oceanodroma melania; and least storm-petrel 0. microsorna). For all species the number of individuals sighted per hour declined by 77 to 94 "'0 over the 3 yr period. This suggests that during ENSO events the Canal de Ballenas may serve as a refugium of high productivity and prey abundance for these highly mobile marine animals. El Nifio-Southern Oscillation (ENSO) events severely reduce primary productivity throughout most of the coastal eastern Pacific (Barber & Chavez 1983, Cane 1983). This causes a decline in the growth, reproduction and survivorship of zooplankton and small schooling fish (Barber & Chavez 1983), and the larger predators that feed on them, including seabirds (Boersma 1978, Schreiber & Schreiber 1983), pinnipeds (Trillmich et al. 1986), and cetaceans (Manzanilla 1989). Some species of highly mobile, long-lived marine animals experience less excess adult mortality during ENSO events than expected from the apparent reductions in localized prey density. Little is known about what measures adults take at sea to compensate for decreased prey density. Mee et al. (1985) proposed that because productivity in part of the central and northern Gulf of California is Present address: Section of Neurobiology and Behavior, Seeley G. Mudd Hall. Cornell University, Ithaca, New York 14853-2702, USA " Present address: 177 Burns Rd. Brooktondale, New York 14817. USA O Inter-Research/Printed In Germany based on tidal current mixing it would not be affected by ENSO events. They suggested this could protect local fauna from ENSO-associated ecological catastrophes. Oceanography of study area and effects of ENSO events. Alvarez-Borrego (1983) and Alvarez-Borrego & Lara-Lara (in press) describe the oceanography of the study area in detail. The canal is narrow and deep (Fig. 1). It undergoes a seasonal change from temperate water conditions (as low as 14 "C) and prevailing northwest winds in winter and spring, to tropical water conditions (as high as 28 "C) with southeast winds in summer and fall. Tidal ranges of up to 4 m generate strong currents (up to 3 m S-'; Alvarez e t al. 1984) which cause extensive vertical mixing (Roden 1964). As a result the canal has the lowest sea surface temperatures (Robinson 1973), and highest nitrate and silicate surface concentrations in the Gulf (AlvarezBorrego et al. 1978). Within the tropical or temperate season surface temperatures appear unaffected by the direction or intensity of wind (Badan-Dangon e t al. 1985). Positive mean sea level anomalies show the onset of the strongest recorded ENSO event in the Gulf of California at the end of 1982 (Robles-Pacheco & Christensen 1984). Mee et al. (1985) used temperature and chlorophyll a time series data from the entrance to the Gulf to show that the ENSO event occurred there from September 1982 through June 1984. Their temperature data indicate enhanced upwelling in 1981 and suppressed upwelling in 1983. In 1981 there was a large standing crop of phytoplankton with average surface value 12 mg m-3. In 1983 there was no large seasonal chlorophyll a maximum and average surface values were < 2 mg m-3. In contrast, Lara-Lara et al. (1984) found that surface nutrient concentrations were not limiting photosynthesis in the Guaymas Basin (just 300 Mar. Ecol. Prog. Ser. 69: 299-302, 1991 south of the Canal de Ballenas) during March and October 1983. In addition, the primary productivity data of Lara-Lara e t al. (1984) are similar to those reported for 1981 and 1982 by Gaxiola-Castro & Alvarez-Borrego (1986) for the same area. Thus, with the scarce data available, El Nino events do not appear to decrease primary productivity in the central Gulf of California where there is intense mixing by tides, and where nutrient concentrations remain high in the euphotic zone. Unfortunately, there are no primary productivity data for 1983 from the Canal de Ballenas, but it has the most intense physical dynamics of the entire Gulf. Therefore, it is where we would expect ENS0 events to have the least effect on nutnent input to the surface waters. Materials and methods. From May through August in 1983, 1984, and 1985 we counted cetaceans and seabirds in the Canal de Ballenas whenever visibility was greater than 5 km and wind speed less than 11 km h-' (Beaufort 2 or less) -about 74 O/O of the days in each year. Because we only conducted counts under these conditions it is unlikely that differences in weather between years affected our ability to sight cetaceans or Fig. 1. Study area seabirds. We used a consistent but non-random search method in which we ran our 4.2 m inflatable boat in a straight line at planing speed (ca 17 km h-') for 5 or 10 min then shut off the engine for 15 min to listen for the blows or exhalations of whales and do a 360" binocular scan. All cetaceans, regardless of distance from the boat, and all seabirds, within an estimated 100 m radius around our boat, were counted. Distance estimates were periodically calibrated using landmarks of known distance. We made no attempt to randomize the search effort on a daily basis. However, on a weekly basis we covered most of the study area (Fig. 1) and all 3 major habitat types (Tershy et al. in press). The relatively small size of the study area and long period of time over which daily counts were averaged (4 mo) minimizes potential interannual differences in area covered. Furthermore, planktivores and piscivores used different parts of the study area (unpubl.) but both groups showed similar changes in relative abundance between years. Observer reliability was high because one of the authors (B.R.T.) was present and consistently collected data on over 95 O/O of the census days. Mixed species feeding aggregations were common Tershy e t al.: El Nif~o in the Canal d e Ballenas 301 and several species had similar seasonal &stributions. However, there was no significant correlation in the daily counts of any 2 species (unpubl.), suggesting that the movements of each species were relatively inde-

Increase in cetacean and seabird numbers in the Canal de Ballenas during an El Nino-Southern Oscillation event