An evaluation of risks to u.s. pacific coast sea otters exposed to tributyltin

摘要

Recent studies have suggested that tributyltin (TBT) and other butyltins may contribute to the deaths of sea offers (Enhydra lutris) off the California coast. Although butyltins have been measured in tissues from the dead offers, nocause-and-effect relationship links these tissue residues to the offer deaths. This study assesses potential risks to U.S. Pacific coast sea offers (in California, Washington, and Alaska) from TBT in their diet. We assumed that bivalves serve as theprimary food consumed. The study looked specifically at how TBT in the diet would affect the species' ability to fight disease (that is, its immune system response). Because no sea offer-specific TBT toxicity data were available, a TBT immune responseendpoint was selected for study in a surrogate mammalian species (with relevant uncertainty factors noted). This endpoint was chosen because effects to the immune system occur at the lowest doses evaluated and because disease has been cited as thegreatest cause of death in California sea offers. Next, the potential for risk to sea offers is estimated for individual sea offers within threatened populations, as well as for the subspecies populations of Alaska, Washington, and California as a whole.Results indicate that no TBT-related risks are expected to any Alaska, Washington, or California sea offers foraging in the coastal waters. These waters represent the habitat where more than 99 percent of the sea offer population is known to reside andforage. A few offers, individuals that have been observed in marinas of Alaska (i.e., Seward, Cordova, and Kodiak) and California (i.e., Monterey Bay area marinas and Morro Bay), may be at risk from TBT exposure if they forage on prey living in themarina. Because the California sea otter population is listed as threatened under the Endangered Species Act, the risks to those individual sea offers using the marina habitat were evaluated using the most conservative scenarios. For example, sea offerswere assumed to feed on marina bivalves for 20 to 50 percent of their diet; it was also assumed that all TBT was 100 percent bioavailable. These assumptions probably overestimate risks, (that is, they are very conservative) because these offers arebelieved to consume more varied prey both inside and outside marinas, and other prey (like crab) may accumulate less TBT in their tissues than do bivalves. Risks are estimated to be negligible to the subspecies populations as a whole, because less thanone percent of sea offers in California, Washington, and Alaska frequent marinas.