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Environmental Research Background What's New

Environmental Research:
Marine Mammals
Sea Otters
Sea Birds
Endocrine Effects
TBT Paint Types

Sea Otter Population Not at Risk from TBT in Their Diet

Researchers (MacLellan et al. 1998) evaluating potential risks to US Pacific Coast sea otters (Enhydra lutris) consuming shellfish containing tributyltin (TBT) found that sea otter populations are not at risk from exposure to TBT. The study indicates shellfish TBT levels are insufficient to cause harmful effects (risks) to the greater than 99 percent of the population that forage in coastal waters. Several toxicity endpoints were evaluated, with immune suppression being the most sensitive endpoint. Sea otters populations were not at risk regardless of the endpoint evaluated.

Tributyltin, a biocide used in ship antifoulant paints, has been suggested as playing a role in the death of sea otters found along the California coast (Kannan et al. 1998). Other researchers (Iwata et al. 1995) have proposed that TBT may have contributed to the deaths of marine mammals through immune system suppression. However, the actual cause of the sea otter and marine mammal deaths has never been determined and several potential causes other than TBT have been suggested. The recent study by MacLellan et al. (1998) indicates that exposure concentrations of TBT are not sufficient to pose potential risks to sea otter populations feeding in the coastal waters of Alaska (see Figure 1), Washington (see Figure 2) and California (see Figure 3).

Sea otters eat primarily bivalves (such as clams and mussels), crab, sea urchin, and abalone (Kvitek and Oliver 1988; Kvitek et al. 1988; Ostfeld 1982). For this study, the sea otters’ diet was assumed to consist solely of bivalves. It was also assumed that all of the TBT in the diet was 100 percent bioavailable. Making these assumptions overestimates TBT exposure because bivalves cannot metabolize (and eliminate) TBT as easily as fish or other invertebrates (Laughlin 1996; Lee 1996). The coastal bivalve TBT data used to assess dietary exposure were obtained from the National Oceanic and Atmospheric Administration, National Status and Trends, Mussel Watch Program, see Figure 2, Figure 3, Figure 4, and Figure 5 (1998). Most (greater than 99 percent) Alaska, Washington and California sea otters live in coastal habitats; thus 100 percent of their diet was assumed to be coastal bivalves.

Because a few individual sea otters in the California and Alaska populations have been observed in marinas, the authors also assessed potential risks to these individuals from TBT exposure in marinas. The marina bivalve TBT data used to assess dietary exposure was collected on a US Environmental Protection Agency-required TBT monitoring program. The authors assumed that a part of the sea otter diet came from bivalves living in marinas, although it could not be confirmed if the sea otters were actually foraging on organisms living there. The remainder of their diet was assumed to be coastal bivalves. Only when it was assumed that the individual otters obtained more than 20 percent of their diet from bivalves actually living in the marinas were they potentially at risk from exposure to TBT. Risk predictions were based on exposures exceeding a moderate immune suppression response. However, when exposure levels were compared to strong immune suppression responses, no risks were predicted for sea otters that may forage in marinas (MacLellan et al. 1998). Risks were negligible to the sea otter population as a whole because less than one percent of California, Washington and Alaska sea otter populations frequent marinas.


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