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California’s Sea Otter Population in Decline
National Wildlife Magazine, June/July 2000

When all the numbers are in , the 1999 spring count tells a worrisome tale--for the fourth year in a row, southern sea otter numbers have gone down. Scientists have been unable to pinpoit the cause: Starvation, disease, pollution, drowning in fishing gear and even El Nino’s storms may all play roles, but none by itself seems enough to explain the problem. “This decline is real, it is significant, and we do not have a simple understanding of why,” says Dave Jessup, a senior wildlife veterinarian with the California Department of Fish and Game. From a peak of nearly 2400 animals in 1995, the southern sea otter count has slid steadily downward year by year - in normal years as well as the stormier El Nino years - until today there are barely 2000 otters left. A bigger concern is that dead animals have been washing up on shore in unprecedented numbers - more than 200 animals in 1998 alone. “We picked up 10 percent of the population dead in 1998. That’s not something that happens very often in wildlife population,” says Jessup. Many of the dead southern otters looked like they were starving, which led some experts to wonder whether the animals were running out of sea urchins, crustaceans, and other bottom-dwelling creatures that form the bulk of their diet. The idea makes sense: Otters are ravenous eaters, gulping down as much as a quarter of their body weight every day.

Despite their enormous appetites, though, Kenner says it’s not clear that otters are dying of hunger. There is little evidence that the otter’s prey species have dwindled in recent years, and there are few signs that hungry otters are migrating up and down the coast in search of better food. Moreover, the wrong animals seem to be dying. “If it was just a food limitation, you’d expect to see the very young and very old dying, but see a lot of prime-aged animals,” says Kenner. The starving animals may be the result, not the cause, of the problem, some scientists think. “It doesn’t take too many days of being too sick to eat to get real skinny real fast,” says Jessup. And indeed, when pathologists examined the recently dead animals, the biggest killer turned out to be disease, which accounted for 40 percent of all deaths. About half of the diseased animals were felled by parasites, especially spiny-headed worm that normally passes from sand crabs into seabirds. If hunger drove more otters to eat sand crabs in recent years, Jessup figures, they may have picked up the parasite in greater numbers than before. The other half of the diseased animals fell victim to an odd variety of problems: massive bacterial infections, a protozoan called Toxoplasma that infects the brain, and even the fungus that causes valley fever, a respiratory disease that affects humans in the Southwest. All three disease organisms are common, yet rarely cause problems in healthy animals. Instead, they tend to pop up when something weakens the immune system. “These three syndromes together suggest there may be something wrong with the immune system of sea otters,” says Jessup.