Tuna - population concerns
As eels only spawn once before death, they require different management to other fish species.
Because of their extensive migrations and long life to maturation, they are a difficult fishery to manage, notably because the relative importance and interaction between habitat, recruitment and fishing pressure have not been quantified. Furthermore, as there is no control on the life stages of tuna while at sea, the restoration of the tuna fishery has had to rely on activities that enhance the population while in freshwater.
The most effective means of conservation is to have sufficient "no-fishing" areas to provide enough eels to reach maturity and spawn. Areas where commercial fishing is not permitted, like National Parks, tend to be inland, often in mountainous areas, and therefore offer limited reserve areas for eels.
Longfins support important customary and commercial fisheries. The commercial fishery is managed through a quota system, and there have been recent reductions in longfin quota in recognition that the eels are vulnerable to over-exploitation. The harvest of eels weighing less than 220 g and more than 4 kg (in the South Island) is not allowed. The upper size limit in the South Island is designed to provide some protection for female eels. Recent management has also focused on increasing the extent of areas where commercial fishing is restricted.
Future management will focus on improved access for both juveniles and adults. Should there be reductions in key indicators like recruitment indices and Catch Per Unit Effort (CPUE), then consideration should be given to reductions in quota and possibly the extension of reserve areas. Although the Japanese eel has been spawned and reared in captivity, production of hatchery-raised Anguilla spp. glass eels in significant quantities is still many years away.
Research has emphasized the longevity and vulnerability of longfin eels to overfishing, and highlighted the need for protected areas. Historically, many hydro dams have not had passage facilities for upstream moving juveniles (December–February) or downstream migrating silver eels (April–June), although steps are underway to rectify this (for example, elver trap and transfer operations are being undertaken at various locations around New Zealand, and new technologies are being trialled for downstream migrants at Wairere Falls Power Station and Karāpiro Dam).
There are no commercial fisheries for migrant eels (silver eels or tuna heke). Any longfin weighing more than 4 kg must be returned unharmed to the water, a law designed to protect potential migrant females. Unfortunately, by itself this regulation is relatively ineffective as it can take 20–30 years for eels to achieve this size, and the likelihood of avoiding capture for this length of time is low.
As male shortfin eels are almost always smaller than the minimum commercial size of 220 g, the shortfin fishery is based almost entirely on female eels. Both male and female longfin are caught, and although the 4 kg upper size limit in the South Island is designed to provide some protection for female eels, the fishery for big eels can be so efficient that few survive long enough to reach this size. Longfin eels were once considered by trout fishery managers as being significant predators of trout, and anglers were encouraged to kill big eels. Today the concern is more for the much-reduced numbers of large eels that remain in many rivers, and there are signs of a localised overfishing.
Longfins are New Zealand's largest freshwater fish, and frequently comprise >90% of fish biomass in streams. They are important apex predators and their removal leads to an increase of prey species. Present commercial harvest levels are c. 150 t (tonnes) per year, and in recent years CPUE appears to be relatively stable in most regions. However, there are ongoing concerns about the wellbeing of stocks due to present recruitment being significantly less than historic levels. In recognition of these criteria and the vulnerability of the species to over-exploitation, longfins are listed as "at risk, declining" in New Zealand's threatened species classification.
Future management of this species will need to be increasingly conservative, with reduced levels of harvest, increased reserve areas, and more emphasis on ensuring the safe downstream passage of silver eels. To ensure long-term tuna conservation and a sustainable fishery, there are research questions still to be addressed in terms of recruitment trends around the country, optimal methods for restocking depleted waters, habitat restoration and enhancement.
References and further reading
Hoyle, S.D., Jellyman, D.J. (2002). Longfin eels need reserves: modelling the effects of commercial harvest on stocks of New Zealand eels. Marine and Freshwater Research 53: 887-895. http://www.publish.csiro.au/paper/MF00020
Graynoth, E., Niven, K. (2004). Habitat for female longfinned eels in the West Coast and Southland, New Zealand. Wellington, Department of Conservation. Science for Conservation 238: 333 p. http://www.doc.govt.nz/upload/documents/science-and-technical/SfC238.pdf
Graynoth, E., Jellyman, D.J., Bonnett, M.L. (2008). Spawning escapement of female longfin eels. New Zealand Fisheries Assessment Report 2008/7. Ministry of Fisheries, Wellington. 57 p. http://fs.fish.govt.nz/Doc/10549/2008%20FARs/08_07_FAR.pdf.ashx
Hitchmough, R., Bull, L., Cromarty, P. (2007). New Zealand Threat Classification System lists 2005. Department of Conservation, Wellington. 194 p. http://www.doc.govt.nz/upload/documents/science-and-technical/sap236.pdf
Jellyman, D.J. (2007). Status of New Zealand fresh-water eels stocks and management initiatives. ICES Journal of Marine Science 64: 1379-1386. http://icesjms.oxfordjournals.org/content/64/7/1379.full
Jellyman, D.J. (2009). Forty years on – the impact of commercial fishing on stocks of New Zealand freshwater eels (Anguilla spp.). American Fisheries Society Symposium 58: 37-56.
Ministry of Fisheries (2009). FRESHWATER EELS (SFE, LFE, ANG). (Anguilla australis, Anguilla dieffenbachii, Anguilla reinhardtii). Tuna. http://fs.fish.govt.nz/Doc/21722/25_EEL_09.pdf.ashx
Potangaroa, J. (2010). Tuna kuwharuwharu, the longfin eel. An educational resource: facts, threats and how to help. 26 p. http://www.rangitane.iwi.nz/education/attachments/169_tuna_vweb.pdf
Townsend, A.J., de Lange, P.J., Duffy, C.A.J., Miskelly, C.M., Molloy, J., Norton, D.A. (2008). New Zealand Threat Classification System Manual. Department of Conservation, Wellington. 36 p. http://www.doc.govt.nz/upload/documents/science-and-technical/sap244.pdf
Ministry of Primary Industries (formerly the Ministry of Fisheries): http://fs.fish.govt.nz/