Over The Horizon

Tracing the Eely Odyssey

As New Zealand's freshwater eels near maturity they leave their rivers and are believed to swim at least 1000km out to sea to spawn. No one yet knows where they go, and Victoria University researchers Peter Castle and Mike Miller are excited at the prospect of finding out.

They have joined a cruise through the western Pacific by the University of Tokyo vessel Hakuho Maru. Finding the spawning grounds for New Zealand's freshwater eels -- a subject of speculation for most of this century -- is one of the objectives.

"With nearly 40 years research on eels of all sorts behind me, nothing could give me greater personal satisfaction than to be involved in such a a discovery," says Castle, who will sail on the leg from Suva to Guam.

Miller, a US National Science Foundation post-doctoral researcher, will sail on a loop starting and ending at Guam.

"The Japanese located the spawning area for the Japanese species in the North Pacific in 1992, and there would seem to be a good chance of success for the New Zealand eels in the South Pacific" he notes.

After eel eggs are spawned and fertilised in the open ocean, the young eels hatch into a distinctive, transparent, leaf-like larval stage known as a leptocephalus (Latin for "narrow head"). All eel species have a leptocephalus, all different in form. They live among the plankton near the surface of the ocean, and the researchers expect to find leptocephali of several different Pacific eels besides the New Zealand ones. One of Castle's interests has been trying to match up the multitude of different eel larvae with the right adults.

Finding the spawning area depends on catching very small leptocephali, just a few millimetres long. If they are this small, they could not have drifted or swum very far from where the eggs have hatched. Large, fine-meshed nets are towed near the ocean surface in the hope that they will catch progressively smaller larvae as the vessel gets nearer and nearer the actual spawning area.

When they reach New Zealand, the leptocephali transform to so-called "glass eels" -- transparent juveniles -- then enter fresh water and soon transform to "elvers". They spend the rest of their lives in fresh water, until the time comes for their final voyage to complete the life-cycle by spawning back at their birthplace in the ocean.

This may seem like a strange way to organise one's life cycle, but Castle says it has advantages.

"The biological gains to eels of such a life history pattern are that the species have a mechanism for adults to spread their young and exploit different habitats. The oceanic young can exploit the rich food supply available in the near-surface layers of the ocean and thus do not compete with the juveniles and adults in freshwaters."

"The main costs presumably are the great losses of offspring on their oceanic journey back to fresh water and the energy costs of a long migratory journey to the spawning area of the mature adults."

There are two species of freshwater eel in New Zealand: the short-finned eel Anguilla australis, which also occurs in southeast Australia and a few other islands; and the long-finned eel Anguilla dieffenbachii which is found only in New Zealand.

Miller and Castle have spent the past six months studying a large collection of leptocephali made some 30 years ago by French scientists from the ORSTOM centre in Noumea, in the same area that will be traversed by the Hakuho Maru.

"There were some freshwater eel leptocephali in the ORSTOM collection but not in sufficient numbers or small enough to enable us to do more than make a well judged guess at the possible location of the spawning area," Castle says.