Nutrition Research
Compared to sea otter populations in other areas, the threatened sea otter population along the coast of central California exhibits slow growth accompanied by a number of other phenomena—such as declining mass-to-length ratios, great diversity in the prey species used by individual animals, increased time devoted to foraging, elevated adult female mortality, and high rates of infectious disease—that are consistent with nutritional inadequacies in otter diets.
Katherine Ralls, a senior scientist at the Zoo, and Olav Oftedal, head of nutrition, are studying the nutritional constraints faced by this population by evaluating the caloric content and nutrient composition of sea otter prey species coupled with studies on otter foraging behavior. By piggybacking this study on to multiyear research efforts using radio-telemetered otters led by scientists at the University of California at Santa Cruz and the U.S. Geological Survey, they will develop the most detailed evaluation of individual variation in diet for any marine mammal and gain an understanding of the impact of diet change on the nutrient intakes of this species.
The nutrient composition of sea otter diets in central California will be compared with that of otters in two healthy, rapidly growing populations: San Nicolas Island, California, and Glacier Bay, Alaska. Sea otters in these areas are larger and fatter than those in central California, spend less time foraging, and consume fewer prey species. Comparative information on the nutrient composition of sea otter diets is essential for determining appropriate management decisions to protect or enhance key nutritional resources that may be limiting the sea otter population in central California.
Radiotelemetry Research
Radiotelemetry is a basic technique for studying wildlife populations but it proved difficult to use this technique with sea otters. Collars did not work as the otters simply removed them. Transmitters attached to tags on the hind flippers lasted only a few months and often fell off.
Sea otter resting on its back on the water.
In the mid-1980s, Zoo researchers and scientists at the University of Minnesota developed a radio transmitter that could be surgically implanted in sea otters and lasted up to two years. They used this transmitter in a pioneering study of the threatened California population, producing new information on many aspects of sea otter biology including movements (Ralls, Eagle, and Siniff 1996), foraging (Ralls, Hatfield, and Siniff 1995), 24-hour activity patterns (Ralls and Siniff 1990), and reproductive and survival rates (Siniff and Ralls 1991).
Sea otters made longer dives, ventured farther offshore and moved over much longer distances than previously suspected. This study was conducted when the California sea otter population was increasing. However, in 1995, the population stopped growing and began to decline.
The reasons for the current decline are unclear, although analysis of over 3000 sea otter carcasses that washed ashore in California from 1968 to 1999 suggests that accidental death in fishing gear may be one problem and that disease is likely contributing to the population's slow growth rate (Estes, Hatfield, Ralls, and Ames 2003). NZP researcher Katherine Ralls is now involved in a large collaborative effort to repeat the earlier radiotelemetry study.
Comparison of the data from this new study with the data obtained from the previous one should provide greater insight into why the California population is declining and the reasons for its chronically sluggish growth rate. This new effort is being led by scientists at the University of California, Santa Cruz.
In addition to an improved radio transmitter, which provides real-time information on core body temperature, sea otters in the new study are also being instrumented with time-depth recorders, which are providing detailed information on the length and location of dives.
