Sea otters, shellfish, and humans: 10,000 years of ecological interaction on San Miguel Island, California

Erlandson, Jon M., Torben C. Rick, Michael Graham, James Estes, Todd Braje, and René Vellanoweth. 2005. Sea otters, shellfish, and humans: 10,000 years of ecological interaction on San Miguel Island, California. Proceedings of the Sixth California Islands Symposium, edited by D.K. Garcelon and C.A. Schwemm, pp. 58-69. Arcata: Institute for Wildlife Studies and National Park Service.

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We use data from San Miguel Island shell middens spanning much of the past 10,000 years in a preliminary exploration of long-term ecological relationships between humans, sea otters (Enhydra lutris), shellfish, and kelp forests. At Daisy Cave, human use of marine habitats begins almost 11,500 years ago, with the earliest evidence for shellfish harvesting (11,500 cal BP), intensive kelp bed fishing (ca. 10,000-8500 cal BP), and Sea Otter hunting (ca. 8900 cal BP) from the Pacific Coast of North America. On San Miguel Island, Native Americans appear to have coexisted with sea otters and productive shellfish populations for over 9,000 years, but the emphasis of shellfish harvesting changed over time. Knowledge of modern sea otter behavior and ecology suggests that shell middens dominated by large red abalone shells-relatively common on San Miguel between about 7,300 and 3,300 years ago-are only likely to have formed in areas where sea otter populations had been reduced by Native hunting or other causes. Preliminary analysis of sea urchin lenses, in which the remains of urchins are unusually abundant, may also signal an increasing impact of Island Chumash populations on kelp forest and other near shore habitats during the late Holocene. Such impacts were probably relatively limited, however, when compared to the rapid and severe disruption caused by commercial exploitation under the Spanish, Mexican, and American regimes of historic times.


In recent decades, the expansion of sea otter (Enhydra lutris) populations along the central California Coast has devastated once productive abalone and sea urchin fisheries that developed in this predator’s absence, creating tensions between resource managers, fishermen, and environmentalists over the protection and management of sea otter populations. Archaeological data from San Miguel Island suggest, however, that Native Americans, sea otters, and productive shellfish populations coexisted on the northern Channel Islands for thousands of years (Walker 1982). These apparently contradictory data sets raise fundamental questions about the nature of “pristine” prehistoric sea otter and shellfish populations, how Native American harvests were sustained over the millennia, the possible ecological effects of sea otter hunting prior to European contact, and the management of modern sea otter populations and healthy marine ecosystems along the California Coast. …

Archaeological evidence suggests that maritime peoples first colonized the northern Channel Islands at least 12,000 to 13,000 years ago (Johnson et al. 2002). On San Miguel, the earliest indication of human occupation comes from a sparse shell midden stratum at Daisy Cave dated to about 11,500 cal BP (Erlandson et al. 1996). From about 10,000 years ago to historic times, we have nearly continuous occupation records from the island’s north coast and we are currently building a 9000 year sequence from the south coast (Braje et al. 2004). San Miguel contains over 600 recorded archaeological sites, including many large shell middens occupied on numerous occasions. For instance, sites SMI-481 and SMI-470, located at or near Otter Point, extend for over 500 m along the coast and contain at least 13 separate occupations dated between about 7,300 years ago and AD 1820. …

… [A]t least 140 discrete animal taxa have been identified, including several extinct species (Erlandson 2004). Artifacts also record the changing technologies that maritime peoples used to harvest the sea and variation in the number and size of dated sites allows rough estimates of the number of people who lived on the island at different times. Two historic Chumash villages are known for San Miguel, with an estimated population in AD 1796 of about 50 people. Archaeological data suggest that Chumash populations on San Miguel were probably considerably larger than this, with several substantial village sites dated to the period just prior to European colonization (Rick 2004).

The first contacts between Europeans and the Island Chumash came in AD 1542, 240 years before the establishment of Mission Santa Barbara, when Spanish ships commanded by Juan Rodriguez Cabrillo wintered on San Miguel Island (Wagner 1929). The Island Chumash may have been devastated by Old World diseases transmitted during such Protohistoric contacts (see Erlandson et al. 2001), but historic accounts suggest that they were thriving when the Spanish first colonized Alta California in AD 1769. Beginning about 1,000 years earlier, Chumash population densities were among the highest recorded for hunter-gatherers anywhere in the world and the Island Chumash were integrated into a regional economy fueled by extensive trade networks, widespread craft specialization, and the use of shell bead currency (Kennett 1998, Arnold 2001). Ethnohistoric accounts indicate that sea otters were highly valued by the Chumash for their furs, which were used to make capes, robes, and blankets (Hudson and Blackburn 1985, King 1990). Along with shell beads, sea otter furs may have been one of the major commodities islanders exported to the mainland Chumash and neighboring tribes. …

Sea Otters and Shellfish In Historical Perspective

… Archaeological data are of variable quality and sample sizes are generally small, but preliminary evidence suggests that sea otters were hunted by the Chumash and their ancestors on San Miguel from at least 9,000 years ago until about AD 1800
(Table 1). …

Despite some gaps in the archaeological record, only a small percentage of San Miguel Island sites have been excavated and dated. It seems likely, therefore, that sea otters were present in island waters throughout the Holocene-until they were eradicated by intensive commercial hunting in the mid-1800s. …

Humans, Sea Otters, and Kelp Forest Ecology

Kelp forests, which grow along many temperate and cool-water coastlines around the world, are some of the most productive ecological communities on earth. In historic times, research around the world has shown that kelp beds are highly susceptible to ecological disruption due to human impacts associated with overfishing, pollution, and other activities (Jackson et al. 2001, Steneck et al. 2002). …

Sea otters and their immediate ancestors have played a key role in structuring kelp forest ecosystems along the Pacific Coast of North America for several million years (Simenstad et al. 1978, Dayton and Tegner 1984, Dayton 1985, Estes and Steinberg 1988). When present, sea otters strongly influence kelp forest food webs, serving as important predators that feed primarily on shellfish in near shore waters. … In North Pacific kelp forests, sea otter predation controls the density and distribution of sea urchins, which feed primarily on kelp. In Alaska and British Columbia, the removal or depletion of sea otters has dramatically effected shellfish populations and kelp forest habitats (Estes and Duggins 1995, Steneck et al. 2002). When sea otters are present, they keep sea urchins in check and limit them primarily to crevices and cracks where they feed on drift kelp. When sea otters are removed or severely depleted, however, urchin populations are released from predation pressure. Expanding rapidly, urchin populations move onto the open ocean floor, feed directly on growing kelp, and can transform three dimensional kelp forest habitats into relatively depauparate and essentially two-dimensional urchin barrens.

… Using archaeological records on the size and abundance of urchin remains found in some Aleutian shell middens, Simenstad et al. (1978) also suggested that heavy Aleut sea otter hunting may have caused such trophic cascades prior to European contact. Dayton (1985) and Dayton and Tegner (1984) argued that Native American hunters may also have controlled sea otter populations along the California Coast, releasing shellfish from predation pressure.

Southern California kelp forests are more complex than their Aleutian counterparts, with more diverse food webs and a wider array of urchin predators, including the California sheephead (Semicossyphus pulcher) and spiny lobster (Panulirus interruptus) (Foster and Schiel 1985, Jackson et al. 2001, Graham 2004). Because of the complexity of California kelp ecosystems, even the near extinction of sea otters in the mid-1800s did not cause a wholesale collapse of kelp forests comparable to those documented in the Aleutians (Dayton and Tegner 1984, Foster and Schiel 1988, Steneck et al. 2002). …

… On San Miguel Island, an apparent increase in the number of urchin-rich midden components through time suggests that humans may have played some role. If such urchin lenses reflect a growing human impact on San Miguel kelp forests, however, dense concentrations of abalones, mussels, and other shellfish in overlying strata at the same sites suggest that such impacts were probably shortlived and localized in scale.

Summary and Conclusions

Archaeological data from San Miguel Island indicate that the Chumash and their predecessors collected mussels, abalones, urchins, and other shellfish from Channel Island shorelines for at least 11,500 years, fished in kelp forest and other near shore habitats for 10,000 years or more, and hunted sea otters and other marine mammals for at least 9000 years. The coexistence of humans with productive sea otter, shellfish, and near shore fish populations on San Miguel through the Holocene-even as human populations, technological sophistication, and trade increased dramatically-raises fundamental questions about the management of similar fisheries along the California Coast today. We have argued that human control of otter populations may have been crucial to the development of a productive red abalone fishery between about 7,300 and 3,000 years ago. Although more speculative, we also suggested that the combination of intensive otter hunting and sheephead fishing may have created localized urchin barrens as sea urchin populations were temporarily released from predatory controls.

If so, current evidence suggests that such smallscale perturbations in San Miguel Island kelp forest communities were limited primarily to the last 3,000 to 4,000 years, when Native populations were larger and more sedentary, and the intensity of near shore fishing was increasing.

This hypothetical model of Native control of sea otter populations might be interpreted as further evidence for resource depression or a “tragedy of the commons” in prehistoric California. It can just as easily be viewed, however, as a case of successful management of nearshore marine fisheries by Native peoples. After all, unlike the European and American hunting that followed them, the Island Chumash appear to have hunted sea otters for millennia, without island-wide extirpation or extinction. On San Miguel, they seem to have done so while maintaining long-term and sustainable harvests of shellfish and fish, despite considerable population growth and technological innovation (Erlandson et al. 2004). Additional study is clearly needed, but it is possible the Island Chumash intentionally enhanced the productivity of local shellfish and fish harvests by controlling otter populations. Such ecological management might logically grow from observations of animal behavior and the changes engendered in local fish and shellfish populations as sea otters were depleted or extirpated from local village territories.

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