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 elements recovered from mainland coastal archaeologica l sites in both California and Oregon. This is surprising as all contempora ry northern fur seals breed exclusively on offshore islands, primarily at h igh latitudes, and the species is otherwise pelagic. The vulnerability of t hese animals to human predation suggests that either humans were foraging m uch further offshore than has been presumed or alternatively that the ecolo gy of these animals has shifted during the late Holocene. We used isotopic and archaeofaunal analysis of the remains of pinnipeds from the middle to l ate Holocene of central and northern California to clarify the breeding and foraging behavior, and migration patterns of these ancient animals. The ca rbon and nitrogen isotope compositions of ancient northern fur seals reveal that these animals fed as far offshore as they do today, and that they rem ained at middle latitudes throughout the year. From an archaeological site at Moss Landing, California, we identified 16 skeletal elements from at lea st 12 very small northern fur seal pups. From another site near Mendocino, California, we identified the remains of at least 6 pups. We estimate the s ize and age of 5 of the young animals using sex-specific regressions of bod y length on the short dentary length derived from measurements of modern sp ecimens. Our estimates indicate these ancient pups were substantially small er, and therefore younger, than modern 3-month-old northern fur seal pups f rom similar latitudes and their nitrogen isotope compositions suggest they had not been weaned. As present-day northern fur seals do not leave their r ookeries until they are at least 4 months old, we consider it highly unlike ly that these ancient pups swam to these mainland locations from some dista nt island rookery. While there are numerous nearshore rocky outcrops along the Mendocino Coast, which may have supported 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 conclud e that northern fur seal adult females, subadults, and pups whose remains w ere recovered at the Moss Landing archaeological site must have been taken at a mainland rookery. Evidence that northern fur seals once bred on the ma inland at this central California location suggests that the abundant remai ns of these animals at numerous other archaeological sites along the Califo rnia coast also reflect the presence of nearby mainland rookeries. Based on the relative abundance of their remains in ancient human occupation sites and the widespread distribution of sites where their remains have been foun d, it appears that northern fur seals were once the predominant pinniped th roughout a region where they now only rarely occur. Furthermore, their pres ence 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.