Southern Sea Otter as a Sentinel of Marine Ecosystem Health

Abstract

The southern sea otter (Enhydra lutris nereis) is listed as “threatened” under the Endangered Species Act (ESA) and is a “keystone species,” strongly influencing the abundance and diversity of the other species within its kelp forest ecosystem. This is accomplished primarily by preying upon urchins that eat the kelp stipe and holdfast, which can reduce a kelp forest to an urchin barren. Sea otters are very susceptible to marine pollutants such as petroleum, which may be directly toxic and/or alter their fur’s insulating properties. Sea otters are an excellent sentinel species. They eat approximately 25% of their body weight per day in shellfish and other invertebrates, and can concentrate and integrate chemical contaminants. In addition, they appear to be susceptible to a number of diseases and parasites that may have anthropogenic origins, and shellfish may serve as an intermediary for some of these infections. Many of the shellfish the otters eat are also harvested for human food. In their role as sentinels, sea otter health has implications for human health, economic sustainability of shellfisheries, as well as overall marine ecosystem health. The recent southern sea otter decline has been viewed with some alarm by conservationists and, indeed, recovery seems a long way off. High mortality rather than depressed recruitment appears to underlie the decline. A good deal of debate has centered on the role of infectious diseases and parasites, exposure to contaminants, nutrition and prey availability, net and pot fishery interactions, and other sources of mortality. Current research is being done related to major classes of mortality, various types of pollutants and some specific organisms causing southern sea otter mortality, and their implications for marine ecosystem health and sustainability.