Environmental Research: • Introduction • Marine Mammals • Sea Otters • Sea Birds • Seafood • Endocrine Effects • Analytical • References Background: • Introduction • TBT Paint Types • Definitions		Questions Remain On The Cause Of Marine Mammal Deaths Introduction Marine mammals die from many causes and their cause of death often remains unknown. In 1987-88, 742 bottlenose dolphins died off the Atlantic coast of the United States (Geraci 1989; Cox 1993; Lipscomb et al. 1994); more than 290 died in the Gulf of Mexico in 1990 (Kuehl and Haebler 1995). A similar event in the Mediterranean Sea resulted in over 1,300 dead and dying striped dolphins washing ashore from July 1990 to October 1991 (Cox 1993). More than 40 sperm whales stranded along Oregon’s Pacific Coast in the 1980s, and many California sea lions became ill and died along the central California coast in 1998. The cause or causes of these mortalities have yet to be conclusively determined. Many possible causes, including brevetoxins from red tide or food containing biotoxins, morbillivirus, mercury, and organochlorines (e.g., DDT and PCBs), have been considered. Recently tributyltin (TBT) has been suggested as a cause for the dolphin deaths occurring in the 1980s and 1990 (Kannan et al. 1997); however, it is not known and is purely speculative that TBT played a role. Kannan et al. (1997) examined the carcasses of 17 stranded dolphins from Florida. These dolphins were not associated with the earlier die-off events in the late 1980s or 1990. Some tissues contained TBT. Kannan et al. did not, however, examine the carcasses for other chemicals (e.g., PCBs). There is no evidence to implicate TBT in the deaths; in fact, many of the dead animals had much lower TBT levels than those found in presumably healthy dolphins and porpoise taken in Japanese fisheries. Interpreting TBT Tissue Levels Caution is needed when making statements that associate tissue concentrations of any chemical with adverse effects on an animal. Neither the Kannan et al. (1997) study nor other TBT studies done in Japan (Iwata et al. 1994, 1995, 1997), were designed to investigate a link between TBT and mortality. Butyltin compounds (e.g., TBT) have also been measured in the tissues of other marine mammals (Tanabe et al. 1998; Kannan et al. 1998; Ariese et al. 1998), and although TBT was found in these marine mammal tissues, it did not provide a measure of adverse effects. Again, other chemicals and previously postulated causes were not measured in the studies. For tissue residues to be effectively linked to effects, a mechanistic relationship with an adverse effect must be established (Chapman et al. 1997). Beyer et al. (1996) suggest that both laboratory and field investigations are necessary to evaluate at what point tissue residue information can be associated with a specific effect. Recent TBT Studies While several studies have attempted to directly link TBT concentrations in the tissues to harmful effects in marine mammals, these studies have been misleading or flawed and have not established a cause-and-effect relationship (Green et al. 1997). In addition, a recent study (MacLellan et al. 1998) of TBT exposure in US Pacific Coast sea otters (Enhydra lutris) indicates risks are not expected to the majority of sea otters (> 99% of the population) from dietary TBT exposure. Finally, many marine mammals forage in offshore waters rather than nearshore habitats. This foraging behavior reduces exposure to TBT because TBT in waters and sediments are very localized to sources such as marinas and shipyards, and areas adjacent to boating activity have been found to have negligible concentrations (Russell et al. 1996; Cardwell et al. 1999; Parametrix, Inc. 1998).
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