Marine biologists now suspect that a mysterious die-off of sea otters in recent years is the result of parasites or infectious diseases that may be linked to pollution streaming into the ocean -- cat feces in particular.
"We are very concerned that the otters are dying so frequently of diseases," said Jonna Mazet, director of the UC Davis Wildlife Health Center and the leader of the center's longtime otter research. "This indicates that the ecosystem is very unhealthy."
A new study by the wildlife health center released Tuesday concludes that endangered sea otters are mostly dying of brain and abdominal infections caused by parasites, as well as a mysterious cardiac disease. But the researchers believe that even otters that die of shark bites are addled by an infectious swelling of the brain that makes them twitch and shake, behavior that can attract sharks.
Furthermore, nearly half of the otters died in the prime of their lives, a disturbing trend for the struggling marine mammal population that halted its rebound in 1995 and has since been either stagnant or in slow decline. About 2,100 California sea otters remain in the wild. Once hunted nearly to extinction for their luxurious pelts, they have been protected by law since 1911.
The UC Davis study surveyed the causes of death found in necropsies of sea otters from February 1998 to June 2001.
It did not include the sea otter carcasses -- 105 as of Tuesday -- that have been discovered on Central California beaches so far this year. But the same patterns of death appear to be continuing this year, said Greg Sanders, sea otter recovery coordinator for the U.S. Fish and Wildlife Service.
"We're seeing the same kind of infections, but just more of it," Sanders said. "We have also seen some boat strikes [killing otters], and the occasional [otter] shootings as well."
In the UC Davis study, headed by wildlife veterinarian Christine Kreuder, 64% of otters died of some form of disease and they appeared to be clustered around Monterey Bay and Morro Bay, the two largest population centers closest to their Central Coast range, which stretches roughly from Point Conception to Half Moon Bay.
One of the most common diseases is encephalitis, or inflammation of the brain caused by a single-cell parasite called Toxoplasma gondii, a parasite only found in cats. Some scientists believe that feces from domestic cats, carrying millions of resilient eggs from these parasites, washes down with storm runoff into the sea, where it is picked up by the otters.
Otters living near sources of storm runoff were three times more likely to test positive for the Toxoplasma parasite, according to an earlier study. This study showed that deaths from this parasite are clustered in Estero Bay, the larger bay that engulfs Morro Bay.
Some of the brain infections have been traced to another parasite, Sarcocystis, which is carried by opossums, a nonnative animal that migrated to the West Coast. Researchers suspect this parasite is also flushed from storm drains and creeks into coastal waters, where otters spend all of their lives.
A parasitic worm also caused a substantial portion of the otter deaths. Scientists believe otters pick up the worm by eating sand crabs. The study showed most of the deaths were clustered on the sandy beaches in the southern end of Monterey Bay.
Researchers are not sure if otters in that stretch of the coastline eat sand crabs simply because the small crustaceans are abundant or because the otters' more typical diet of sea urchins, larger crabs, lobster and abalone is less available due to overfishing.
Another cause of otter mortality, cardiac disease, surprised researchers by claiming as many otters as shark attacks. A few died of valley fever, which is spread through the spores of a soil fungus found in dust.
Other studies, still underway, are focused on establishing precisely how marine mammals contract land-based diseases.
Researchers in Santa Cruz, for instance, are trying to determine if man-made contaminants washing into the ocean decrease otters' immunity, leaving them more vulnerable to parasites, as some scientists suspect.
"Otters are often referred to as the 'sentinel species,' " said Sanders, the federal otter recovery coordinator. "On the land, we can see our influence on the landscape, with homes and roads. But when it gets to the ocean, it's less obvious. These [otter] diseases show we have a profound influence in the marine environment, in terms of water flowing into it and what's in the water."