Keeping tabs on our native eels
New tags that are attached by a tough nylon thread could finally solve the mystery of where New Zealand‘s freshwater eels spawn.
4 July 2001
Scientists from NIWA are using the tags – which can record environmental information and transmit it to satellites – to study the spawning migration of New Zealand‘s longfin eels. The project is a joint venture with the University of Tokyo, which paid for the tags at a cost of $9000 each.
The new tags are smarter than your average tags. If the eel remains in the sunlit surface layer – where light can penetrate – the tags can calculate the eels‘ position using the sunrise and sunset to estimate longitude, and day-length to estimate latitude. At a preset time the tag will then detach from the eel and float to the surface, where it will send an estimate of the eels‘ position, including hourly records of temperature and depth, to the satellites. The path of the eel can then be plotted on a map.
And what if the eel dies? NIWA fisheries scientist Dr Don Jellyman says they have already considered this possibility.
“Each tag has a small, pressure sensitive guillotine, so if the eel dies and sinks to the bottom, the guillotine will automatically cut the tag off at depths of more than 1000 m. The tag will then float to the surface instead of falling to the seabed and being lost.”
Dr Jellyman says scientists have been intrigued about the breeding grounds of freshwater eels for decades.
“The spawning grounds of eels have traditionally been found by towing fine meshed nets at sea and then back-tracking to find the smallest larvae. Unfortunately, we still have not found any larvae from our native longfin eel.”
“What we do know is that freshwater eels are tropical marine species that have invaded fresh water, so the breeding areas of all species are likely to be in the tropics.”
Interest in discovering the breeding grounds of freshwater eels is largely academic, says Dr Jellyman, because it would be impractical to try to capture the delicate larvae for eel farms. However, there may be important implications for conservation. For example, if the spawning areas were found in the open ocean, as expected, protection of the eels would require an international agreement.
Last month NIWA scientists attached the tags to 10 longfin eels (weighing between 6 and 10 kg), which were collected by commercial eel fishers from Lake Ellesmere, south of Christchurch. The eels were then acclimatised to salt water and released into the open sea.
“We set the tags to detach at monthly intervals from August to December, which gives us a position on the map each month so we can track the path of the migrating eels.”
There are 15 species of freshwater eel in the world, but the spawning grounds of only three temperate species have been discovered – the European, American, and Japanese eel. New Zealand has three species of freshwater eel – the shortfin, the Australian longfin, and our native longfin.
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