We investigated the impact of foraging location (nearshore vs offshore) and foraging latitude (high vs middle) on the carbon (δC) and nitrogen (δN) isotope compositions of bone collagen of northern fur seals (Callorhinus ursinus), harbor seals (Phoca vitulina), California sea lions (Zalophus californianus), and northern elephant seals (Mirounga angustirostris). Nearshore-foraging harbor seals from California had δC values 2.0‰ higher than female northern elephant seals foraging offshore at similar latitudes. Likewise, nearshore-foraging harbor seals from Alaska had values 1.7‰ higher than male northern fur seals, which forage offshore at high latitudes. Middle-latitude pinnipeds foraging in either the nearshore or offshore were C enriched by ∼1.0‰ over similar populations from high latitudes. Male northern elephant seals migrate between middle and high latitudes, but they had δC values similar to high-latitude, nearshore foragers. Female northern fur seal δC values were intermediate between those of high- and middle-latitude offshore foragers, reflecting their migration between high- and middle-latitude waters. The δC values of California sea lions were intermediate between nearshore- and offshore-foraging pinnipeds at middle latitudes, yet there was no observational support for the suggestion that they use offshore food webs. We suggest that their "intermediate" values reflect migration between highly productive and less-productive, nearshore ecosystems on the Pacific coasts of California and Mexico. The relative uniformity among all of these pinnipeds in δN values, which are strongly sensitive to trophic level, reveals that the carbon isotope patterns result from differences in the δC of organic carbon at the base of the food web, rather than differences in trophic structure, among these regions. Finally, the magnitude and direction of the observed nearshore-offshore and high-to middle-latitude differences in δC values suggest that these gradients may chiefly reflect differences in rates and magnitudes of phytoplankton production as well as the δC value of inorganic carbon available for photosynthesis, rather than the input of C-enriched macroalgal carbon to nearshore food webs.

The remains of northern fur seals (Callorhinus ursinus) are among the most abundant of pinniped elements 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 species is otherwise pelagic. The vulnerability of these animals to human predation suggests that either humans were foraging much further offshore than has been presumed 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 carbon and nitrogen isotope compositions of ancient northern fur seals reveal that these animals fed as far offshore as they do today, and that they remained at middle latitudes throughout the year. From an archaeological site at Moss Landing, California, we identified 16 skeletal elements from at least 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 size and age of 5 of the young animals using sex-specific regressions of body length on the short dentary length derived from measurements of modern specimens. Our estimates 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 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 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 location suggests that the abundant remains of these animals at numerous other archaeological sites along the California 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 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 ...

Zooarchaeological data from Monterey Bay and the adjacent central California coast corroborate earlier observations from northern California and Oregon that Callorhinus ursinus (northern fur seal) was a much more common component in prehistoric marine mammal prey than its present pelagic distribution and foraging habits would predict. C. ursinus remains from mid-Holocene Monterey Bay occurrences are predominantly from female individuals, associated with an inshore piscifauna, and lack associated artifactual evidence for deep water exploitation. Taken together with evidence from Oregon, this suggests that mid-Holocene C. ursinus populations had different foraging, resting, and, arguably, reproductive behaviours than historically reported. Currently debated is whether the contrast between prehistoric and present patterns of pinniped species representation results from: 1) late Holocene prehistoric resource depression by aboriginal hunters, 2) depredations of the early historic fur trade, or 3) non-anthropogenic climatic or oceanographic change. The issue has thus far been addressed with presence or absence data on pinniped species and age/sex classes in dated contexts. While these are fundamental data, they cannot shed light on the nature of foraging behaviour of the species in question, a key dimension of the problem. This paper reports a pilot study utilizing stable isotope analysis aimed to elucidate prehistoric pinniped foraging patterns, specifically that of C. ursinus. Elements from six archaeological sites in Monterey and Santa Cruz counties were analysed for stable isotope compositions of carbon and nitrogen in bone collagen and compared to a latitudinally ordered modern dataset. Results for archaeological C. ursinus strongly suggest that prehistoric animals habitually foraged at lower latitudes than characterize the species today, supporting earlier claims of their year-round residency south of Alaska.

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