Dolphin deaths: a tributyl tin connection?

Added to marine paint, the compound tributyl tin checks the growth of barnacles and algae on the bottom of boats. In the late 1980s, many countries, including the United States, banned its use on small hulls after researchers began documenting the toxic effects of the so-called antifouling compound on other marine creatures, such as oysters and mollusks.

In 1995, researchers reported that dolphins found dead along Japanese coasts had accumulated a variety of butyl tin compounds, the breakdown products of tributyl tin. In the January Environmental Science & Technology, a team of researchers headed by Kurunthachalam Kannan, now at Michigan State University in East Lansing, reports the first measurements of butyl tin accumulation in dolphins from U.S. coastal waters.

Over the past decade, dolphins along the Atlantic and Gulf Coasts, as well as in the Mediterranean Sea, experienced several mass die-offs involving hundreds of animals. Kannan and his colleagues took tissue samples from bottle-nosed dolphins that had died along the Florida coasts. The butyl tin concentrations they measured, notably in the liver, were generally higher than those in stranded dolphins from other locations and in a captive dolphin. Liver tissue from a smaller sample of two other species that range farther out to sea-the pygmy sperm whale and the Atlantic spotted dolphin-had butyl tin concentrations one-third to one-fourth those of the bottle-nosed dolphins.

The finding doesn't prove that tributyl tin killed the coastal-dwelling bottle-nosed dolphins, but there is other evidence that butyl tin compounds are potent immune system suppressors. The researchers suggest that the pollutants may have diminished the animals' ability to fight off the bacterial or viral infections thought to underlie the deaths. Other industrial pollutants that accumulate through the food chain have also been implicated in die-offs of marine mammals (SN: 7/2/94, p. 8).

The researchers noticed that butyl tin was lower in dolphins that died in 1994 than in those that died in earlier years, before the U.S. ban would have had an effect. A similar pattern has turned up in Gulf oysters. Exposure will continue because tributyl tin persists in sediments and is still allowed on large vessels and aluminum hulls, the researchers note.