Satellite tracking should help us understand our eel species. PHILIP ENGLISH reports.
The mystery of where New Zealand's native longfin eels spawn is about to be revealed through satellite tracking.
The eels, slimy, repulsive and menacing to some, but impressive for their lifespans of 40 to 50 years - even up to 100 years in some specimens - are thought to migrate to the tropics to spawn.
National Institute of Water and Atmospheric Research scientist Dr Don Jellyman is determined to find the proof.
Four months ago 10 eels fitted with tags allowing them to be tracked on their Pacific Ocean migration were released near Lake Ellesmere, south of Christchurch, in a joint research project between Niwa and the University of Tokyo.
The tags, costing $9000 each (and paid for by the Japanese), collect data on the eels' position and depth as well as water temperature. At pre-set times the tags detach from the eels, float to the surface and transmit their data through satellites to Dr Jellyman via Australia.
Provided the eels keep swimming in the ocean's photic zone, where light penetrates the water, the tags calculate the eels' positions by recording the time of sunrise and sunset so their longitude and latitude can be estimated.
In late August Dr Jellyman received data from the first two tags showing the position of one eel about 200km east of Gisborne and the other eel about 250km east of Christchurch.
Dr Jellyman said that between now and late December the remaining eight tags would pop to the surface revealing the eels' positions and eventual general spawning area.
"I really can't say anything on the basis of the information we have from the tags so far but my educated guess is it will still be in the tropics probably to the east of Fiji and Samoa. It may be quite a long way further east than that but it will probably be at that sort of latitude."
Longfin eels, endemic to New Zealand, do not feed during their migration north where they spawn and die. Females that grow up to 120cm long can have ovaries the size of 50 per cent of their body, containing 15 to 20 million eggs. After spawning with smaller males the eel larvae return to New Zealand freshwater rivers drifting haphazardly on ocean currents.
"They have no prior knowledge of where the adults came from ... We have confirmed that by some work we have done on DNA content of eels from locations throughout the country.
"If there was some homing activity going on or there were juveniles returning to the waters where their parents had left from, then something would start to show up in stocks that we could pick up through DNA differences."
Dr Jellyman said the eels were adapted to long-range oceanic migration but their distribution depended on current systems.
"There are bits of the jigsaw puzzle we do not understand but we are gradually piecing stuff together.
"What is important is that we monitor the recruitment of juvenile eels coming back to know there is sufficient spawning going on to maintain good stocks."
Dr Jellyman said the life story of longfin eels should make people think twice before they set out to catch and kill them.
Longfin eels are one of New Zealand's most common freshwater fish and are found throughout the country but large specimens are now rare.
"There was a prevailing philosophy that the only good eel was a dead one and they were bad for trout stocks and that sort of thing. There was quite an active programme going back 40 or 50 years to eradicate them which is thankfully no longer in place.
"When you are dealing with a creature that is considerably older than you are, especially for the big female longfins, you should have some respect for it."
* The world has 16 known species of freshwater eel, occurring in Europe, the east coast of North America and throughout the eastern Pacific and Indian Oceans.
* Three species are found in New Zealand - the longfin eel, the shortfin eel, and the Australian longfin recently found in the Waikato River and believed to have colonised other river systems.
* Longfin eels have a longer dorsal fin than anal fin. Shortfin eels have smaller mouths and dorsal and anal fins almost the same length. Australian longfins have black blotches on their backs and sides.