Tuna - elvers and recruitment
Once in freshwater, glass eels develop into darker pigmented juvenile eels known as elvers.
Elvers of both longfins and shortfins are common found in the swiftly flowing gravelly rapids and riffles, where they live and feed amongst the gravel. Elvers migrate upstream, when temperatures reach about 16°C, usually between late November and early March.
These migrations are particularly visible at instream obstacles, like waterfalls and hydroelectric dams in the lower reaches of rivers, where elvers can often be found at night wriggling up any damp areas. They are able to climb vertical surfaces until they are about 12 cm long, at which size they are too heavy to stick using surface tension. These congregations of elvers may last for several weeks, during which time they are heavily predated on by larger eels, trout, rats and shags. During this time, they are also collected by people, for transfer to other locations upstream.
Keep an eye out in your local waterway during summer for elvers travelling up your stream. At dusk use a strong torch to spot them at places where they are trying to climb over waterfalls, dams and overhanging culverts. If you see large numbers having a hard time trying to get over an instream structure, talk to your local Council. There are ways of making this a bit easier for our native freshwater fish.
The past few decades have seen a decline in the recruitment of Northern Hemisphere species of freshwater eels. There are indications that the huge elver runs that were both witnessed and filmed in New Zealand prior to the 1960s are no longer seen, and that current runs are considerably less than historical runs.
In New Zealand and around the world, the climbing ability of elvers has been exploited in the design of elver ladders and trap and transfer (also called trap and truck) facilities. Most high dams in New Zealand are now equipped with eel ladders and/or have trap and transfer operations. The trap and transfer of elvers above obstacles is not a new idea - Māori have long assisted tuna to pass over obstacles such as waterfalls.
It is not yet known exactly how present recruitment relates to historical runs, but since 1995 the Ministry of Primary Industries (previously the Ministry of Fisheries) has been using trap and transfer operations at key sites around New Zealand to establish trends in elver recruitment. The numbers of elvers captured and transferred at each site varies between location and year, but the most productive site - Karāpiro Dam on the Waikato River - provides around two million elvers annually for seeding upstream. On average, since the 1999–00 season, about four million elvers are transferred from the base of dams to upstream habitats in New Zealand each year.
Continued monitoring of eel recruitment around New Zealand is essential, as indications from population models, and experience from overseas eel stocks, are that large declines in recruitment can happen within a few years. At present the Ministry of Primary Industries contracts NIWA to collate and analyse all of the recruitment information (e.g., numbers, species compostion, timing) being collected around the country at large instream barriers. Unlike many overseas countries who have decades worth of records, New Zealand's datasets are fairly "young" and it is important that we continue to collect this nationally important dataset over the long term.
References and Further Reading
Best, E. (1929). Fishing Methods and devices of the Maori. Dominion Museum Bulletin No 12. 92 p.
Boubée, J., Jellyman, D. (2009). Facilitating the upstream and downstream passage of indigenous fish at large dams. In: E. McSaveney. (Ed.). Dams – operating in a regulated Environment. IPENZ Proceedings of Technical Groups, 35/1: 57-63.
Boubée, J., Chisnall, B., Watene, E., Williams, E., Roper, D., Haro, A. (2003). Enhancement and Management of Eel Fisheries Affected by Hydroelectric Dams in New Zealand. In: D. A. Dixon (Ed). Biology Management and Protection of Catadromous Eels, pp. 191-205. American Fisheries Society, Bethesda, Maryland, USA.
Hayward, R., Hayward, R. (1992). Eel history was a mystery. Auckland, Haward Historical Film Trust. 18 mins.
Jellyman, D.J. (1977). Summer upstream migration of juvenile freshwater eels in New Zealand. New Zealand Journal of Marine and Freshwater Research 11: 61-71. http://www.tandfonline.com/doi/pdf/10.1080/00288330.1977.9515661
Martin, M., Stevenson, C., Boubée, J., Bowman, E. (2009). Recruitment of freshwater elvers, 1995–2009. New Zealand Fisheries Assessment Report 2009/58. Ministry of Fisheries, Wellington. 44 p. http://fs.fish.govt.nz/Doc/22122/09_58_FAR.pdf.ashx