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Mercury contamination found in stranded dolphins

18 June 2008

Research by Honours student Alissa Monk has revealed high mercury levels may be a contributing factor to dolphin deaths in Victorian waterways.

Monash University research into heavy metal contaminant levels in dolphins from Port Phillip Bay and the Gippsland Lakes has revealed high mercury levels may be a contributing factor to dolphin deaths.

Researchers from the School of Biological Sciences Dr Thompson have found that the mercury is likely to have come from the sediments of the bay and are concerned that dredging activities may increase the dolphins’ exposure.

The confirmed levels of mercury found in the dolphins were within a range considered to cause negative health and mental effects and were higher than mercury levels found in populations around the world.

Over the last two years Honours student Alissa Monk has measured mercury concentrations in 20 live dolphins and eight animals that died after becoming stranded.

Supervisory researcher Dr Ross Thompson said levels in the dead dolphins averaged 3.45 milligrams of mercury per kilogram of tissue compared to 1.32 mg/kg in living dolphins.

"Mercury levels detected are sufficient to cause significant health impacts and were comparable to those found in areas of the world that are considered highly polluted, including the Mediterranean Sea," Dr Thompson said.

Mercury has been shown in previous national studies to accumulate in dolphins, but this is the first study to find particularly high levels in stranded animals in coastal Victoria.

The contamination is concentrated by the process of bio-accumulation whereby smaller fish containing mercury are eaten by larger mercury contaminated fish that are then consumed by dolphins. Dolphins can consume up to ten kilograms of fish per day.

"Dolphins may be becoming stranded as a direct consequence of mercury contamination which damages their neurological system," Dr Thompson said.

"They become potentially confused and disorientated and strand themselves.

"Even the apparently healthy dolphins had high levels of mercury which put them at risk of future health complications."

"Sediment contains mercury likely to have originated from historical gold mining sites as well as from other industrial sources," Dr Thomson said.

"Over time the mercury has been washed down through waterways, including the Yarra River, and come to rest on the bottom of the bay."

Dr Thompson said it was critical that further studies were undertaken throughout the bay dredging process to ensure any further decline in dolphin health could be identified and managed.