Heavy metal poisoning could be causing dolphin beachings

Australian scientists have found that mercury poisoning could be leading local dolphins to beach themselves. Ross Thompson and Alissa Monk at Monash University in Melbourne suggest that the mercury’s neurological effects would include confusion and disorientation, causing the dolphins to strand themselves on the shore.

While dolphins are known to accumulate mercury through the fish they eat, Thompson’s study is the first to compare mercury levels in beached dolphins with live animals from the same population. Thompson sampled dolphins living in the shallow, enclosed waters of Port Phillip Bay and the Gippsland Lakes in Victoria, Australia, finding that the beached dolphins averaged 3.45 milligrams of mercury per kilogram of body weight, over twice that of the live animals.

’The mercury levels we found in the dead dolphins were high enough to be causing quite severe neurological effects,’ Thompson told Chemistry World. ’Even the levels in the apparently healthy population would be expected to cause immune deficits, at the very least,’ he adds.

"The mercury levels we found in the dead dolphins were high enough to be causing quite severe neurological effects" - Ross Thompson

The findings confirm earlier European studies on harbour porpoise populations, says marine ecotoxicology expert Krishna Das of Liege University in Belgium. But Das adds that it is very difficult to assess the effects of a single pollutant. ’Mercury could be a contributing factor to the dolphin deaths - but it is never acting alone as the animals have high levels of a lot of other pollutants,’ she says.

Mining connection

Historical gold mining, which up until the 1930s used mercury in gold extraction, is the likely main source of the heavy metal in the waters, says Thompson. The mercury has gradually washed down the Yarra and other rivers, and accumulated in the sediments of the bay. Current dredging work could further increase mercury levels in the food chain, he adds. The mercury levels he measured were already higher than those found in dolphins in other populations in polluted waters around the world, from the Mediterranean to costal India.

As well as following the changing mercury levels as dredging continues, Thompson plans to examine how today’s mercury levels compare with those of the past. ’We will test mercury levels in the teeth of museum specimens, which include dolphins from the late 19th century to the very recent, to see how mercury accumulation has changed over time,’ he says.

As well as dolphins, Port Phillip Bay is also home to a colony of Fairy Penguins, but their mercury accumulation wasn’t so high, probably because they feed lower in the food chain, and don’t live as long. ’The penguins seem to be hit by zinc instead - the other main pollutant - but that’s another story,’ says Thompson.

James Mitchell Crow