Declining native eel population causes concern
Forty years ago anglers in New Zealand were actively encouraged to ‘kill eels on sight’. Although the attitude towards eels has changed significantly since then, it has not stopped our native longfinned eel stocks declining, mirroring the global downward trend in freshwater eel recruitment.
Freshwater fisheries principal scientist Don Jellyman from NIWA said there is increasing evidence to show that current levels of commercial fishing and habitat loss could seriously affect the sustainability of our longfinned eel fishery.
His comments are timely because of the Third National Wildlife Conference opening in Christchurch today, where the sustainability of New Zealand’s native species is being discussed. In New Zealand’s classification system for native species longfinned eels are described as chronically threatened because their numbers are expected to decline by 5 to 30 per cent in the next decade.
Dr Jellyman, who returned from an international symposium on freshwater eels in Canada in August, said our longfinned eels are particularly vulnerable to over-fishing because they take a long time to grow, usually only between 1 and 2 cm a year. ‘This means female eels can be very old – up to 80 years old – by the time they migrate out to sea to spawn. Because of their slow growth they’re vulnerable to commercial capture for many years before they eventually spawn.’
‘Our population models indicate that the current upper size limit for longfin female eels of 4kg is unlikely to be effective because many eels will be captured before they reach this size. For example, if a female eel grew 3 cm a year it would spend an average of 21 years in the fishery before reaching the 4kg limit.’
Unlike northern hemisphere countries, New Zealand has no commercial fishery for baby eels (called glass eels), which arrive in from the sea each year during winter and spring. As a result, there is no long-term data on their recruitment. Other indices, such as computer population models and catch per unit effort, are therefore used to determine any changes in recruitment over time.
‘Our concerns for the longfin eel are based on a number of factors, including the very low numbers of juvenile longfins in some catchments, their slow growth rates, the significant declines in catch per unit effort, and the reduction of or loss of access to waterways because of hydrodams. The number of populations dominated by males, especially in the lower half of the South Island, which supports the largest longfin eel fishery in the country, is also of particular concern.’
The North Island commercial eel fishery is currently non-regulated, but is due to follow the South Island fishery and become part of the Quota Management System in 2004. Recreational fishing regulations, which were introduced in 1994, allow a daily bag limit of six eels.
‘Eels are very difficult to manage because they spawn once and then die. The most effective conservation and management strategy is to ensure that enough eels are able to migrate out to sea each year to spawn. The 9 to 10 months that eel larvae spend at sea is something we can’t control.’
‘Historically, longfin eels have been our most frequently encountered freshwater fish, but today it’s becoming increasingly difficult to find large females. Given the long generation times of this species, it may be many years before we know what the full effects of habitat loss and over-fishing are.’
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