|
|
|||||
|
Sancutary Program Accomplishments Open Ocean & Deep Water Systems Endangered & Threatened Species Bird Populations
|
|
In Central California, we studied the response of seabirds to El Niño at Southeast Farallon (SEFI) and Año Nuevo islands (ANI). ANI is home to the Rhinoceros Auklet, a relative of puffins, and a species which nests in underground burrows and approaches the colony only under the protective cloak of darkness. SEFI is located about 90 km north of ANI, beyond the boundaries of the Monterey Bay National Marine Sanctuary. This colony supports core populations of Brandt's and Pelagic Cormorants, Western Gulls, Common Murres, Cassin's Auklets, and Pigeon Guillemots in Central California. During the non-breeding season, individuals disperse to the north and south, thus bringing SEFI birds into the Monterey Bay region. While seabird breeding at these sites in 1997 was relatively unaffected by El Niño, it was apparent early on that things would be different in 1998. (See Figure 1 for examples.) From March to June, mean sea surface temperatures at SEFI were 1 - 2.5° C higher than long-term (27-year) averages. By July and August, however, temperatures had dropped to average values. Warmer than normal winter and spring ocean temperatures, and the corresponding drop in ocean productivity (i.e., prey for birds) resulted in moderate to substantial delays in egg-laying. For example, the mean Common Murre and Cassin's Auklet egg-laying dates were delayed by two and eight weeks, respectively. Breeding populations were also much reduced. Again, considering the murre and auklet: the SEFI murre population was about 20 percent lower than in 1997, while the auklet population was almost 65 percent smaller. Similarly, numbers of breeding Pigeon Guillemot declined by over 75 percent, Brandt's Cormorant by about 35 percent, and Pelagic Cormorant by roughly 50 percent. For those that attempted reproduction, success was very poor. Pelagic Cormorant and Pigeon Guillemot experienced near total reproductive failure. The reproductive performance of Brandt's Cormorant was 30 percent of the 27- year mean, whereas for the murre, this value was 52 percent, and for Rhinoceros Auklet it was 22 percent. The only species which demonstrated near normal productivity was Cassin's Auklet, in which the value for 1998 exceeded the long-term mean by 16 percent. However, given that very few Cassin's Auklets attempted to breed, island-wide offspring production was extremely low. In short, these observations were quite similar, with slight variations, to observations made on the Farallon seabird community in other severe El Niño years.  Our time series for Rhinoceros Auklets on ANI is considerably more limited, and efforts to enhance this population by providing habitat (nest boxes) may complicate interpretations. Nevertheless, El Niño's influence was apparent there as well. While the auklet population increased 98 percent from 1993 to 1997 (due to management efforts), it declined by 18 percent in 1998. Mean productivity for pairs in nest boxes also increased from 0.33 to 0.64 young/pair between 1993 and 1997, yet dropped to 0.47 young/pair in 1998. In natural burrows, productivity dropped from an average of 0.83 young/pair to 0.55 young/pair. Lastly, we noted a major change in the chick diet for Rhinoceros Auklets on ANI. In past years, northern anchovies composed the vast majority of the prey items brought to developing offspring, whereas in 1998 Pacific saury made up most of the diet. Saury generally occur further offshore than anchovies, requiring greater foraging effort by adults. This mechanism may help explain reduced productivity in this species on ANI in 1998. El Niño, and its counterpart, La Niña, generally persist on time scales of one to two years; however, the biological effects of El Niño may be further complicated by oceanographic change which operates on longer time scales. In particular, the 1998 El Niño has come at the end of a period of general ocean warming. Thus, while El Niño serves to illustrate one endpoint in the natural range of variation, it is only applicable to the climate regime under which observations are obtained. Few El Niños studied by marine ornithologists occurred during a cool-water climate regime. This highlights the need for continuing long-term observations to evaluate El Niño/La Niña effects under both warm and cold water climate regimes. As highly-visible upper trophic level predators, birds can be used as accurate and immediate gauges to the timing and intensity of both relatively short and longer-term oceano-graphic anomalies. William J. Sydeman |
|||
