Elver trap and transfer

The New Zealand native longfin eel, also known to Māori as ‘ tuna’, is one of the largest and longest-living freshwater eels in the world.

In order to spawn, longfin tuna migrate to deep-sea trenches in the Pacific Ocean, up to 6,000 kilometres off our coastline. They release millions of eggs, and their offspring float back to the New Zealand coast on ocean currents. The elver (young tuna) then swim upstream to live in freshwater rivers and lakes. One of the major obstacles that tuna and elvers face on this journey is man-made structures such as dams.

The Waiau and the Waitaki catchments are the natural habitat of thousands of native tuna, and building and operating dams in these areas has had impacts on their migratory habits. Meridian and Ngāi Tahu consider the tuna population a key indicator of the quality of these waterways.

As the tangata whenua of the area, Ngāi Tahu has historically relied on a healthy tuna population for mahinga kai. We recognise the cultural importance of eel to Ngāi Tahu and work with them closely to ensure the protection of the species and ensure that this taonga (treasure) is preserved for many generations to come.

To provide a sustainable population of eel in the Waiau and Waitaki catchments, we move thousands of tuna each year by trapping and transferring the elver into dam headwaters and migrating adults back downstream. These processes involve Ngāi Tahu and other local stakeholders in overseeing and delivering the trap and transfer programme.

In the Waiau catchment, the programme involves physically trapping female migrating longfin tuna in Lake Manapōuri and transferring them to below the Manapōuri Lake Control structure – from here they have open access downstream, enabling them to migrate successfully to sea for spawning. The programme also transfers elver upstream into Lake Manapōuri. This programme involves local stakeholders and has delivered good results, with high levels of female eel migrants, which is an important factor when it comes to successful breeding.

Within the Waitaki catchment we work with Arowhenua, Moeraki and Waihao rūnanga to support and facilitate the movement of longfin elver and migrant tuna. The number of elver that we catch has been variable since we began our trap and transfer programme. This is due to a number of factors that we are working with others to better understand.

We have also worked in partnership with the National Institute of Water and Atmospheric Research (NIWA) to track the migratory behaviour of 220 young and adult tuna in Lake Manapōuri and Waitaki in the past few years. This research has informed us in choosing the preferred trap and transfer tuna option to use in the Waiau catchment to help enhance efforts to protect the native species.

We will continue with the trap and transfer programme as a way to help protect longfin tuna and will continue to monitor regularly the effects and review the best way to manage eel populations.