|Breeding-plumage Rhinoceros Auklet. Photo by Jukka Jantunen (VIREO).|
We recently published a paper1 on the prey selected by these diving seabirds, in relation to the timing of their breeding and the growth and survival of their offspring. We also examined ocean-climate influences, to learn whether effects were different for birds breeding in two different settings.
In the California Current (from British Columbia to Baja California), seabirds are top predators in a highly productive ecosystem. From plankton to porpoises, life here is concentrated at every level of the food web. This is especially true in years when strong upwelling draws nutrients upward from deep water, chilling the sea surface temperature (SST) in the process. Cold-water years generally correlate well with high prey abundance and with seabird breeding success, but the ecosystem may vary from year to year.
|Figure 1. (Click on caption for large version.) Auklet diet is an indicator of juvenile rockfish abundance. Numbers on vertical axes are mathematical measures of rockfish..|
We have found that Rhinoceros Auklets ("Rhinos") forage for the most readily available and nourishing fish or squid species they can find. At our island study sites, we can identify the foods that parent birds bring their chicks. Our data show that the auklet's diet changes in composition from year to year—in close parallel with changes found in fish samples from trawling surveys done by the National Marine Fisheries Service (NMFS).
Figure 1 plots these trends for one prey species, juvenile rockfish—very important among the small species that marine predators use for food. Rockfish, anchovies, herring, and the like are the so-called "forage fish"—crucial in the transfer of energy up the food web. Forage fish consume plant and animal plankton (sometimes smaller fish) and, in turn, are consumed by top predators such as seabirds and salmon. The close parallels in Figure 1 show that auklets are strong indicators of the marine ecosystem.
|Figure 2. (Click on caption for large version.) Breeding success varies between the two islands. The vertical axis shows number of chicks fledged per pair. (Rhinoceros Auklets lay one egg.).|
We also examined the links between SST, prey abundance, and auklets' reproductive success. In summary, when sea surface is cold, Rhinos find plentiful rockfish and their chicks survive well to fledging. Figure 2 shows one component of this pattern, offspring survival to fledging. It correlates well with another component (not shown), abundance of food carried by adult birds to their chicks.
Our study also revealed a distinction between the two islands. The influence of upwelling (cold SST) is more pronounced for Rhinos at the Farallones (solid line in Figure 2), situated near the edge of the continental shelf, than it is at Año Nuevo, only 90 km from the Farallones but much nearer the coast. For conservation management, this is valuable information: it demonstrates that the impact of physical and biological factors can vary in relation to geographic region, even on a very small scale.