An evaluation of risks to US Pacific coast sea otters exposed to tributyltin

Recent studies have suggested that tributyltin (TBT) and other butyltins may contribute to the deaths of sea otters (Enhydra lutris) off the California coast. Although butyltins have been measured in tissues from the dead otters, no cause-and-effect relationship links these tissue residues to the otter deaths. This study assesses potential risks to U.S. Pacific coast sea otters (in California, Washington, and Alaska) from TBT in their diet. The authors assumed that bivalves serve as the primary 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 otter-specific TBT toxicity data were available, a TBT immune response endpoint 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 the greatest cause of death in California sea otters. Next, the potential for risk to sea otters is estimated for individual sea otters 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 otters foraging in the coastal waters. These waters represent the habitat where more than 99 percent of the sea otter population is known to reside and forage. A few otters, individuals that have been observed in marinas of Alaska and California, may be at risk from TBT exposure if they forage on prey living in the marina.