Tracking Eels Across the Oceans
Somewhere in the warm waters of the Pacific, possibly around Fiji or Samoa, there is a mysterious place where all of New Zealand's freshwater eels go to breed. Nobody knows exactly where. A project involving scientists from the New Zealand National Institute of Water and Atmospheric Research (NIWA) and the University of Tokyo has been trying to unravel the mystery.
Background
New Zealand has two species of freshwater eel: Longfin eels (Anguilla dieffenbachii) and shortfin eels (Anguilla australis). The longfin eel is found exclusively within New Zealand; the shortfin eel is more widespread, and is found around southeastern Australia and other southern Pacific islands. Although both species can occur together, longfin eels are more common in inland waters; shortfin eels are more common near the coast.
When they mature at around 35-40 years, female longfins leave their homes and head for the sea. The breeding instinct is so strong the eels will leave water and navigate their way around obstacles, such as dams and weirs, blocking their path. The eels go through several changes before leaving: their heads become more streamlined, their eyes enlarge (with a blue ring appearing around them) and their gut degenerates, resulting in loss of appetite. The Māori have a special name for migratory eels, Tuna-heke, as they cannot be caught using bait because of this lack of appetite.
In a journey lasting months, the eels swim north, sometimes in warm surface waters, sometimes in the cold and dark, almost a kilometre down; when they reach the spawning grounds, they mate with longfin males who made the same journey slightly earlier and release as many as 20 million free-floating eggs. Then they die. Shortfin females make the same journey when they reach 20-25 years.
Eels hatch as leptocephali – small, transparent, leaflike larvae, which bear little resemblance to adults. These drift and swim about, making daily trips up and down the water column, feeding on plankton. After about 7-8 months, shortfin eels metamorphose into round-bodied young eels called elvers. Longfin larvae take a little longer before changing – about 10 months. Then, somehow, the tiny eels make their way to New Zealand. On reaching the coast, they swim up rivers and streams, feeding on bottom animals until they become black-and-silver-bodied adults, completing the cycle.
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