Holocene changes in the ecology of northern fur seals: insights from stable isotopes and archaeofauna.

The remains of northern fur seals ( Callorhinus ursinus) are among the most abundant of pinniped ele- ments recovered from mainland coastal archaeological sites in both California and Oregon. This is surprising as all contemporary northern fur seals breed exclusively on offshore islands, primarily at high latitudes, and the spe- cies is otherwise pelagic. The vulnerability of these ani- mals to human predation suggests that either humans were foraging much further offshore than has been pre- sumed or alternatively that the ecology of these animals has shifted during the late Holocene. We used isotopic and archaeofaunal analysis of the remains of pinnipeds from the middle to late Holocene of central and northern California to clarify the breeding and foraging behavior, and migration patterns of these ancient animals. The car- bon and nitrogen isotope compositions of ancient north- ern fur seals reveal that these animals fed as far offshore as they do today, and that they remained at middle lati- tudes throughout the year. From an archaeological site at Moss Landing, California, we identified 16 skeletal ele- ments from at least 12 very small northern fur seal pups. From another site near Mendocino, California, we identi- fied the remains of at least 6 pups. We estimate the size and age of 5 of the young animals using sex-specific re- gressions of body length on the short dentary length de- rived from measurements of modern specimens. Our es- timates indicate these ancient pups were substantially smaller, and therefore younger, than modern 3-month- old northern fur seal pups from similar latitudes and their nitrogen isotope compositions suggest they had not been weaned. As present-day northern fur seals do not leave their rookeries until they are at least 4 months old, we consider it highly unlikely that these ancient pups swam to these mainland locations from some distant island rookery. While there are numerous nearshore rocky out- crops along the Mendocino Coast, which may have sup- ported small breeding colonies, the Moss Landing site is centered on a 40-km-long sandy beach, and is more than 120 km from what at the time were the nearest offshore islands. We conclude that northern fur seal adult females, subadults, and pups whose remains were recovered at the Moss Landing archaeological site must have been taken at a mainland rookery. Evidence that northern fur seals once bred on the mainland at this central California loca- tion suggests that the abundant remains of these animals at numerous other archaeological sites along the Califor- nia coast also reflect the presence of nearby mainland rookeries. Based on the relative abundance of their re- mains in ancient human occupation sites and the wide- spread distribution of sites where their remains have been found, it appears that northern fur seals were once the predominant pinniped throughout a region where they now only rarely occur. Furthermore, their presence along the central and northern California coasts appears to have once severely limited the distribution of other pinnipeds, which are now common to the region.