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Migrating eels give clues to pollution

eel bone - UTS

Ringing the changes: a cross-section of the otolith of an adult eel, showing age rings. The holes have been drilled to sample the bone. (Pic: Marine Ecology Labs, University of Technology Sydney)

The unusual life cycle of the long-finned eel could turn out to be a valuable pollution monitoring tool, Australian researchers have found.

The ear bones of the eels, like those of other fish, show growth rings like those of a tree. Called 'otoliths', the bones can be analysed to indicate changes in the surrounding water going back in time.

"Eels live for 50 to 60 years and their otoliths can give us a history of what has occurred in that time," said Associate Professor David Booth, marine ecologist with the Department of Environmental Science at the University of Technology, Sydney (UTS).

"This is a measure of the amount of pollutants that are actually getting into the fish, rather than how much is in the water, and that is what is relevant for human health."

Dr Booth and his team are investigating the eel's migration and breeding patterns to establish whether it is sustainable to harvest the highly lucrative glass eel - the transparent baby form of the long-finned eel (Anguilla reinhardtii).

Eels are a delicacy in Japan and Europe, where they usually eaten smoked. There is also a growing market for them as a restaurant dish in Australia.

Japan has a strong history in aquaculture, but the long distances swum by these eels make their life cycle difficult to reproduce.

To get around this, the Japanese harvest their brood stock from the wild and grow them to adult size in tanks. But a mixture of over-harvesting, pollution, and damming of rivers has made this unsustainable, and Japan is now turning to Australia for brood stock.

Glass eels sell in Japan for more than $1000 a kilo, and there have been rumours of a black market operating at around $10,000 a kilo, said Dr Booth.

Currently, however, the legal harvest in Australia is very small, with only two people in New South Wales issued licences to harvest 20 to 30kg of glass eels annually.

Working with New South Wales Fisheries, the UTS researchers want to find out whether harvesting the babies threatens adult eel stocks, which are among the most valuable in the estuarine fishery market.

The researchers track the eels' movements by looking at their otoliths - first removing the bone from the fish, then running samples through a spectrometer to analyse the elements.

The ratio of strontium to calcium in each sample indicates whether the eel was living in salt water or fresh water.

"If the strontium level is high, then it was in salt water, and if it is low, then it was in fresh water," Dr Booth explained.

His results show that the adult fish swim 6000km out to sea to spawn, from New South Wales rivers all the way to the Coral Sea, near the Solomon Islands.

"Right in the centre of the bone the strontium levels are very high, which indicates that spawning occurred in the sea," he said.

The eels spawn billions of baby glass eels - called leptocephali, meaning leaf larvae, because they are flat like a leaf.

"They waft or swim back to the coast of Australia, New Caledonia, New Zealand and New Guinea to float back into the rivers and grow out into adult eels."

Genetic work by a collaborating team of researchers will determine whether the eels in each country are the same species or slightly different.

For ecologists, the eels' migration patterns provide a window on past environments.

"You can't sample them in the ocean because you rarely find them," said Dr Booth.

"They are like little canaries in mines."

Physiologists are also interested in the transformation from a freshwater to a saltwater fish. Salmon move the other way, from salt water to fresh water.

Tags: oceans-and-reefs, biology, marine-biology