Population Biology of the California Sea Otter

Past sea otter populations once ranged across the Pacific Rim from northern Japan to Baja California. Some 15,000 to 20,000 animals occurred in California alone. But like so many species, sea otters were overexploited. A mere fifty or so individuals remained near the mouth of Bixby Creek in 1911 when federal law prohibited further hunting.

A five-year study is underway that will help to identify threats to California sea otter populations. photo MBNMS

This surviving colony gradually increased in abundance and range, albeit at a perplexingly slow rate. Then, in the mid-1970s, perplexity turned to alarm as the California sea otter population took a downturn. The likely cause of the decline was determined to be incidental mortality in fishing nets. Once this problem was identified and redressed, the California sea otter population began to increase – a trend that continued until the mid-1990s, when numbers again began to decline. Annual population surveys indicate that sea otter numbers in California currently are either stable or declining.

The sluggish population growth during the best of times and declining numbers in recent years still mystify marine biologists and natural resource managers. To make matters worse, little research has been done on California sea otters since the late 1980s. At the most fundamental level, the depressed population growth is thought to be a consequence of elevated mortality, not diminished reproduction or immigration.

Why are so many otters dying? At least three explanations have been proposed, including elevated levels of infectious disease, nutritional limitation, and incidental mortality in fishing gear. Information from necropsies of beach-cast carcasses is insufficient to explain the high levels of mortality because only about half of the deaths are recovered as beach casts, and the cause of death cannot be determined for many of these.

The obstacle to understanding mortality in California sea otters has been a lack of funds rather than a lack of ideas, but that situation has now changed. New federal funding for research on California sea otters was provided in 2001, owing to the hard work of numerous local people, a research grant from the Minerals Management Service, efforts by Representative Sam Farr to obtain Congressional support, and a renewed commitment to learning by the responsible state and federal agencies.

The planned study, which began earlier this year and involves scientists from the U.S. Geological Survey, California Department of Fish and Game, University of California (Santa Cruz and Davis campuses), and the Monterey Bay Aquarium, will take five years. Its centerpiece is the comprehensive study of marked animals at three main sites: the Monterey Bay area, near Cambria, and south of Point Conception. About fifteen animals will be captured at each of these sites during both of the first two years of the study. These animals will be equipped with flipper tags, surgically implanted radio transmitters, body temperature sensors, and—in some animals—surgically implanted time-depth recorders. The instruments will give scientists a window into how sea otters live and die, and health profiles will be taken from each animal at the time of capture. The study will contrast these various measures with equivalent measures taken in a similar study conducted in the mid-1980s—a time when the population was increasing. Our study design will provide intriguing contrasts between the center of the current range (where food presumably has been depleted) and ends of the range (where food is more abundant).

Besides mortality, which is the metric of direct concern, diverse data gathered from the tagged otters will permit a more in-depth assessment of population status. For instance, if food is limiting, we would expect to see changes in diet, foraging behavior, and body condition between the ends and center of the range and between the mid-1980s and present. The importance of infectious disease should become much clearer as health profiles from the living animals become available. Similarly, the significance of incidental mortality should become evident from the fate of tagged animals. The physiological studies will provide a more basic understanding of how these small marine mammals cope with the rigors of life in a cold ocean (e.g., how and when they are most susceptible to cold; whether or not deep diving poses a thermal challenge to them; the costs and benefits of feeding on different prey species; and the energetic costs of diving).

Modeling provides a final dimension to the project. Various models are being developed to understand better how the observed patterns of behavior, physiology, and demography play out as long-term trends in abundance and distribution.

Long-term survival of the California sea otter depends in part on identifying and redressing significant threats to the population. The results of this study will provide an important step toward that end.

James A. Estes(1), M. Tim Tinker(2), Tom W. Williams(2), Dave Jessup(3), and Andrew B. Johnson(4)
(1)Biological Resources Division, United States Geological Survey
(2)Department of Biology, University of California Santa Cruz
(3)California Department of Fish and Game
(4)Monterey Bay Aquarium

Marbled Murrelet Research in the Sanctuary

The Marbled Murrelet (Brachyramphus marmoratus) is a small seabird in the puffin family that breeds along the west coast of North America from the Aleutian Islands south to the Santa Cruz Mountains. Unlike other seabirds, Marbled Murrelets breed inland in old-growth redwood and Douglas fir forests. The Santa Cruz Mountains population is geographically separated from the next major population, in Humboldt County, by more than 300 kilometers. Population numbers are difficult to estimate for this enigmatic bird, but there are probably about 1,000 birds in the local population. Marbled Murrelet populations have been declining throughout the species’ range, due primarily to loss of nesting habitat. This decline led to the listing of the species by the State of California as endangered, and by the U.S. Fish and Wildlife Service as threatened.

Marbled Murrelet populations have been declining, due primarily to loss of nesting habitat. photo Esther Burkett

The Marbled Murrelet has been notoriously difficult to study. The first nest was not found until 1974, here in the Santa Cruz Mountains. Marbled Murrelets don’t make much of a nest; they usually lay a single egg directly on a large horizontal limb of an old-growth tree. The tree limb must be large and flat enough to prevent the egg from rolling off, and the tree canopy must provide some cover from nest predators like Steller’s Jays (Cyanocitta stelleri). This choice of nesting habitat, usually in the top third of very tall trees, makes studying the bird difficult.

Recent research in and around the Monterey Bay National Marine Sanctuary is helping to elucidate some of the mysteries of the Marbled Murrelet. Research on the local population falls into three categories: dawn forest surveys, at-sea surveys from boats, and radio-telemetry.

For more than a decade, local researchers have been conducting dawn surveys in forests of the Santa Cruz Mountains. During the nesting season, from May to August, adults take turns incubating for twenty-four hours at a time, switching duties at dawn. After the egg hatches, adults deliver a daily meal of fish for the nestling, usually at dawn. By monitoring activity levels in different forest habitats, researchers can determine which areas are most likely to harbor nesting Marbled Murrelets and track long-term changes in activity levels.

At-sea surveys during the nesting season have been conducted locally for several years by Ben Becker and others at the University of California Berkeley (UCB). Because of the difficulty of effectively counting the birds at their nesting sites, at-sea surveys are the best tool to estimate population size. During the summer months, adult Marbled Murrelets are mottled brown (see photo), but the juvenile birds are black and white, resembling tiny Common Murres (Uria aalge). By carefully estimating the ratio of juvenile birds to adult birds on the water, researchers at UCB have been able to model the structure of the local population. At-sea surveys conducted during the non-breeding season by Moss Landing Marine Laboratories have helped determine the year-round distribution of the species. The local population generally moves south out of Año Nuevo Bay after they molt in September, and birds in their black and white winter plumage can usually be seen offshore from Santa Cruz in October. From November to April Marbled Murrelets occur primarily off Aptos in northern Monterey Bay, and then in April the birds return north to Año Nuevo and molt into their brown breeding plumage.

In 1997 a radio-telemetry study was initiated by Esther Burkett (California Department of Fish and Game) and was subsequently taken on by researchers at UCB. These studies have been instrumental in locating several nest sites in the forest and have increased our understanding of the species’ nesting ecology tremendously. Interestingly, in the El Niño year of 1998 no radio-marked birds flew inland to nesting habitat, indicating that they may have been too physically stressed to breed.

One of the most interesting results of the radio-telemetry studies was the discovery that although the foraging range of breeding Marbled Murrelets was less than twenty-five kilometers, several birds dispersed from Año Nuevo to the south end of the Sanctuary, near Pt. Piedras Blancas—a journey of 200 kilometers. The birds were presumably traveling a considerable distance for some predictable food source.

Little is known of the at-sea distribution or seasonal occurrence of Marbled Murrelets in this area. The uncertain future of several offshore oil leases south of the Sanctuary off Morro Bay creates a pressing need for knowledge of Marbled Murrelet habitat use at the southern end of their at-sea range. Research on this unique species is continuing on several fronts in the Sanctuary, and through our increasing knowledge, we may be able to stem the rapid decline of the species.

Laird Henkel
Moss Landing Marine Laboratories