Variable responses of seabirds to change in marine climate: California Current, 1985-1994

Abstract

We conducted annual ship-board surveys to determine the density and distribution of seabirds off central California in relation to marine climate, from 1985 to 1994. Summarized here are results for the sooty shearwater Puffinus griseus, the common murre Uria aalge, and Cassin's auklet Ptychoramphus aleuticus, the 3 most abundant seabirds in the central part of the California Current (91% of seabird abundance and biomass). During the study, sea-surface temperature, wind speed and thermocline depth all increased, salinity decreased and thermocline intensity (slope) showed no consistent trend. Periods of cool water and warm water, as well as offshore and inshore excursions of the shelfbreak front, alternated and were mediated by the Southern Oscillation and upwelling intensity. The responses to climate variation by the 3 seabird species were in accord with their respective morphologies and natural history patterns. All moved closer to shore and away from the shelf-break front (which also weakened). Abundance of the shallow-diving shearwater and auklet decreased dramatically, but those of the deeper-diving murre did not. The shearwater, which nests in the southern hemisphere and is the most mobile of the 3 seabirds, likely changed its migration routes and reduced its association with the California Current. The auklet, which breeds within the study area and lacks mobility, declined in number, most likely due to reduced breeding success and subsequent reduced population size. Remaining auklets moved away from the shelfbreak, but not as far inshore as the shearwater. The murre, which also breeds locally, is the most adaptable owing to its deeperdiving capabilities. It shifted distribution within the study area to feed on alternative prey found throughout the water column.

Physical and biological variables affecting seabird distributions during the upwelling season of the northern California Current

“…Our estimates, with the exception of the slope habitat, were similar and resulted in nearly an identical total (May: 45.0, 32.0, and 2.3 kg km -2 ; total = 79.3 kg km -2 ). Given that the Sooty Shearwater, by far the most abundant species in the CCS (and which contributes immensely to overall biomass), declined by 90 % in the CCS since 1976 (Veit et al, 1997;Oedekoven et al 2001;Ainley and Divoky, 2001), our results are surprising. We expected to encounter significantly fewer numbers and biomass than was present, as we have noted in central California studies (e.g., Oedekoven et al, 2001).…”

“…Given that the Sooty Shearwater, by far the most abundant species in the CCS (and which contributes immensely to overall biomass), declined by 90 % in the CCS since 1976 (Veit et al, 1997;Oedekoven et al 2001;Ainley and Divoky, 2001), our results are surprising. We expected to encounter significantly fewer numbers and biomass than was present, as we have noted in central California studies (e.g., Oedekoven et al, 2001). It is possible that in addition to their overall decline in the CCS, the shearwaters have become more confined to those regions that still provide high food availability.…”

“…In a study at a similar scale in the eastern tropical Pacific, but using a larger study area and only physical and temporal variables (e.g., SST, SSS, depth and slope of thermocline, date), Spear et al (2001) were able to explain just 8.3% and 20.7%, respectively, of the variance in the abundance of planktivorous and piscivorous avian species. At a spatial scale similar to the present study but with 12 years of data from one season (April-June), a study in the central CCS that considered most of the habitat variables but not the biological ones in the present study, Oedekoven et al (2001) explained 52-58 % of variance in the abundance of Sooty Shearwater, Common Murre, and Cassin's Auklet. Finally, using broadly spaced surveys over the entire CCS from Washington to Mexico (to 300 km offshore), but using only distance to the center of thermally-defined fronts (dramatic gradients in SST) as the independent variable at a 4-km scale (n = 55 fronts), Hoefer (2000) was able to explain occurrence variance as follows: Sooty/Pink-footed shearwaters combined (57 %), Leach's Storm-Petrel (3 %), phalaropes combined (52 %), Western Gull (85%), Common Murre (85 %), and Cassin's Auklet (6 %).…”

“…Hoefer, 2000;Oedekoven et al, 2001), this is the first to examine the effects of biological variables simultaneously with the former. Indeed, the CCS, as one the world's four most productive (boundary current) ocean systems and host of a highly abundant and diverse seabird community, is especially well suited for a study of the processes and cues used by seabirds when searching for prey.…”

“…Oceanographic variables examined (sea-surface temperature and salinity, thermocline depth and structure) are those shown previously to have a major effect on seabird distributions at sea (e.g. Hoefer, 2000;Oedekoven et al, 2001Hyrenbach and Viet, 2003); spatial variables are the predictable front locations in the study area, and biological variables are those representing primary productivity and prey abundance. We tested two hypotheses: 1) that bird distributions were most strongly affected by biological variables, particularly prey abundance, compared to physical variables; and 2) that within-season evolution of the upwelling features (alongshore jet, etc.…”