Over The Horizon

Closing In on the Eels

The mystery of where New Zealand's eels go to breed [, Sep] is nearly solved.

Associate Professor Peter Castle and Dr Mike Miller of Victoria University, working with an international team of scientists on the Japanese research ship Hakuho Maru, searched for young eels of several species by trawling nets with very fine mesh near the surface of the Pacific. The smaller the eels they were catching, the nearer they knew they were getting to the spawning area.

It is not yet possible to draw definite conclusions from the voyage's findings, but Castle is excited about the information gained and believes he and his colleagues are not far away from solving the long-standing mystery of the eels' spawning area.

"I think we've got the location of the spawning grounds for the South Pacific species fairly well pinned down," he says. "It seems to lie between 10 and 15 degrees south of the equator, between Samoa and Fiji but more towards Samoa, in an area of relatively low ocean salinity. This water may be the trigger to spawning that the mature adults need."

"The smallest ones captured were less than 20mm long, which means they were not much more than two weeks old," says Castle. "Given the known speed and direction of the current, which flows strongly east to west at the latitude where we collected the specimens, they must have been spawned just a few hundred kilometres further to the east."

"The young eels are extremely hard to identify. There are six species in the South Pacific and we've narrowed down some of the 140 larvae to about three species, including the two New Zealand species. We will have to carry out DNA work now to identify these specimens more precisely, to see whether they really are the young of the New Zealand species."

From the spawning grounds, Castle believes, the young eels are carried westwards by the currents, south of the Solomons and north of New Caledonia, before encountering the narrow East Australian current and being swept back to New Zealand, a process estimated to take perhaps as much as a year. After spending around two to three decades here living in fresh water, adult eels then undertake the long return trip in order to breed.