Physical and biological determinants of the abundance, distribution, and diet of the Common Murre in Monterey Bay, California
Croll, D.A. (1990) Stud Avian Biol 14:139-148
Physical and biological factors affecting the diet, distribution, and abundance of the Common Murre (Uria aalge) in Monterey Bay, California were investigated from September 1981 through September 1983. Murre diet shifted both seasonally and annually, indicating an opportunistic feeding strategy. Highest abundance of murres was found during the summer period of late upwelling when murres exploit a dependable peak in prey availability (juvenile rockfish, Sebastes spp.) resulting from earlier upwelling episodes. Murres probably use this peak in food availability to feed dependent chicks at sea, replenish fat stores, and molt. During fall and winter productivity in Monterey Bay is low, and its importance to murres is reduced.
Distribution during summer when murre abundance is high is probably determined by local upwelling and current patterns. Densities were highest in the northern region of Monterey Bay, probably due to higher food availability. Water is advected from the southern to the northern portions of the Bay, carried by an eddy of the California Current. Upwelling is centered off of Point Pinos to the South. As recently upwelled, nutrient rich water is transported from south to north, it promotes increased phytoplankton production, which works its way to higher trophic levels as it is carried north. This results in higher prey availability in the north, and thus higher Common Murre density.
The primary effect of the 1982/1983 El Nifio-Southern Oscillation phenomenon was a decrease in primary productivity that lead to a reduced availability of the normally dependable summer prey resources. As a consequence, murres which came into the Bay in June 1983 in large numbers quickly dispersed, resulting in low densities in July and August. Murres that were found in Monterey Bay at this time were thin and fed on a different array of prey items.
This study supports the hypothesis that concentrations of higher trophic level marine predators are concentrated “downstream” from upwelling centers. Peak abundance of murres in Monterey Bay occurred shortly after the seasonal peak in upwelling. During this peak abundance, murres were concentrated in the northern portion of the Bay (Soquel Cove) which is downstream of the upwelling center off of Point Pinos to the south.
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