As wildlife agencies release a revised plan to protect southern sea otters, a sudden increase in fatalities raises concerns about the future of the species
Conservationists saw the long-awaited release of the U.S. Fish and Wildlife Service revised southern sea otter recovery plan in February as an important step toward ensuring the long-term survival of the species. The good news, however, was soon tempered by reports of 44 sea otter fatalities in April—more than double the 10-year average for the month.
Since January, more than 100 southern sea otters, nearly 5 percent of the 2,000 aimal population, have washed up dead on California shores, according to state officials. Officials from the California Department of Fish and Game and researchers from the University of California-Davis are conducting necropsies on the recovered animals to determine what is causing the spike in fatalities.
"There are no clear patterns in causes of death, location of stranding, or age structure," said Greg Sanders, the southern sea otter recovery coordinator for the FWS. "We're trying to put together the pieces."
High rates of fatality have plagued southern sea otters for several years. The population has fallen 10 percent since its peak of 2,200 in 1995, said Dr. David Jessup, senior wildlife veterinarian at the California Department of Fish and Game. With such a all population, sudden spikes in fatalities could have aignificant impact on the long-term viability of the species.
Dr. Jessup explained that if the otter population shrinks, its federal species listing status will be changed from a threatened species to an endangered species.
Finding an explanation for the deaths will take time. It may take up to six months to complete the necropsies because of the unusual number of deaths, according to Dr. Jessup. The process could be fast-tracked if the FWS Working Group on Marine Mammal Mortality Events declares the deaths an "unusual mortality event" and provides additional funding. Sanders said the final decision had not yet been reached at press time.
Searching for answers
The FWS recovery plan calls for more research to understand the factors limiting southern sea otter population growth, and for actions to reduce or eliminate threats to the population, Sanders said. Human interference with otters, oil spills, ocean contamination, infectious diseases, and conflicts with commercial fishermen are some of the threats targeted in the plan.
A PDF version of the recovery plan is available at http://pacific.fws.gov/ecoservices/endangered/recovery/. A print version is available from the Field Supervisor, Ventura Fish and Wildlife Office, 2493 Portola Road, Suite B, Ventura, CA 93003.
Wildlife veterinarians are taking an active role in otter population recovery efforts by helping to identify the medical causes of sea otter fatalities. Previously, recovery efforts focused on reducing risks from oil spills, boat collisions, and other highly visible signs of trauma.
"It's a new approach," said Dr. Jonna Mazet, a veterinarian, wildlife epidemiologist, and director of the Wildlife Health Center at the University of California-Davis.
Dr. Jessup agreed, explaining that wildlife population management has usually been driven by nonmedical factors.
"There are not many instances where health and disease issues drive wildlife population management," he said. "Veterinarians and biomedical wildlife health professionals have not always been at the table."
In the case of the southern sea otter, however, infectious diseases appear to be an important factor limiting population growth.
"There may be some type of immune suppression," Sanders said.
The March 15 issue of JAVMA highlighted a study by UC-Davis researchers that examined the link between freshwater runoff and Toxoplasma gondii infection in California sea otters, to view the story visit https://www.avma.org/News/JAVMANews/Pages/030315e.aspx.
A recent epidemiologic analysis of 105 otter fatalities from February 1998 through June 2001, conducted by researchers at the UC-Davis Wildlife Heath Center, found unusually high numbers of otters dying from disease in their prime breeding years. According to Dr. Mazet, 64 percent of the otters examined died of disease and an unusually high rate of death in breeding age adults was noted.
The analysis will be published in the July issue of the Journal of Wildlife Diseases.
The analysis was led by Dr. Christine Kreuder with Dr. Melissa Miller, one of the primary investigators in the T gondii study, as a chief collaborator. The analysis found four primary causes of death:
- T gondii was responsible for 16 percent of the fatalities
- Infection with thorny-headed worms (acanthocephalans) caused 16 percent
- Heart disease caused 13 percent
- Shark attacks caused 13 percent
Many of the sea otters that suffered fatal shark bites had preexisting T gondii brain infections, a condition that may have made the animals less able to evade attack, more likely to venture far from shore into sharks hunting areas, and prone to erratic movements that might attract sharks, Dr. Mazet said.
Additionally, the researchers found geographic patterns of disease. Dr. Miller's study found a high proportion of T gondii infection in sea otters near sites where freshwater runoff empties into the ocean, raising concerns about water contamination. Infection with acanthocephalans was also found in a cluster near the southern end of Monterey Bay.
Dr. Mazet said the UC-Davis Wildlife Health Center, the California Department of Fish and Game, the Monterey Bay Aquarium, and the U.S. Geological Survey would continue to gather data on sea otter deaths through necropsies and will compile and analyze the data collected to track fatality trends over the long term.