Scientists have long been mystified by the slow growth of the California sea otter. Sea otter populations in California have straggled along at a growth rate of 5-7% at best. There have been precipitous declines, slow growth periods, and unusual mortality events with extreme numbers of otters washing up dead on beaches. Right now, the sea otter population is entering another worrisome decline.

A population decline or slow growth can be attributed to either slow or impeded birthrates, or an elevated mortality. Scientists believe that elevated mortality is the main reason for slow growth of the sea otter population. (Estes et al, 2003) This means more sea otters are dying annually than are being born to replenish and increase the population. Sea otters were listed as a threatened species because of the risk of an oil spill wiping them out--yet other, more subtle dangers are slowly wearing the population down.

What's killing California's sea otters?

Disease

More than 50% of studied otter mortality is attributed to disease of some sort. Certain diseases are more prevelent than others, and otters in different locations are more susceptible to different diseases. Recent research (Prey choice and habitat use drive sea otter pathogen exposure; Martin et al, 2009) suggests that the make-up of an individual otters' diet can influence that animal's susceptibility to certain types of diseases. What is clear is that otters are washing up infected with diseases to which they are not native hosts. Two common examples are toxoplasma gondii, a parasite whose primary host is the cat, and sarcocystis neurona, a parasite that infects horses and possums.

California sea otters in the Central Coast are swimming in polluted water, accumulating heavy chemicals such as DDT, PCBs and PBDEs. According to a 2008 study, DDT is the predominant chemical contaminant found in California sea otters, followed by PCBs. It has been concluded that "marine mammals from the California coast contain some of the highest DDT concentrations ever reported." (Kannan et al, 2008)

The same study notes that "disease, pollution and starvation can all influence sea otter mortality, [but that] the precise factors responsible for the recent decline in the sea otter population are still unclear." (Kannan et al, 2008)

Many believe that heavily contaminated California sea otters are suffering immune effects from poor water quality that leaves them more susceptible to pathogens, many of which are introduced to the marine environment.

The Otter Project supports continued research into sea otter pathology and the impacts of chemical pollution on otters. The Otter Project also believes that the ongoing degradation of the marine environment as a result of poor watershed management and land use practices are contributing substantially to the decline in otter health.

In 2007 the Monterey Coastkeeper, a program of The Otter Project, was established to address water quality in the Central Coast, much of the sea otters' range. Two of the Coastkeeper's founding projects were to address urban runoff and agricultural pollution.

Oil

Sea otters were listed as a threatened species largely out of concern for the potential impacts a single, catastrophic oil spill could have on the California sea otter population. This fear was substantiated by the tragic loss of thousands of sea otters as a result of the Exxon Valdez spill in the Prince William Sound, Alaska. Nearly twenty years later researchers believe that otters in the Sound are still experiencing chronic health affects from the disastrous spill. The Otter Project still considers oil a threat to otters, and continues to address oil spill prevention and response by ensuring that otters are considered in vessel traffic management, opposing offshore oil drilling, and emerging concerns in spill response.