Eel Enhancement

New Zealand has always had natural obstructions to eel passage. 


Maori have a long history of using a ‘kete’ to carry elvers over these obstructions and thereby allowing tuna (eel) to access the habitat above the obstruction and thrive.

In more recent times, man has built dams, weirs, culverts, perched culverts, drains, flood gates, flood pumps and all manner of obstructions which hinder freshwater fish in their quest to find a place to live. 




Some elvers might indeed be able to leave the water and climb over an obstruction but their numbers are decimated in doing so – akin to soldiers making an attack in WW1!

waterfall as an obstruction to eel passage

By the late 1980's eel habitats were being seriously impacted by land developments, flood pumps, dams and many other issues.  These impacts pressured the eel industry and others to look at solutions. It was very apparent that migrating elvers were dying in massive numbers on the spillway of karapiro  dam and a working group consisting of Tainui, industry and ECNZ  resolved to conduct catch and capture of elvers for transfer into  lake karapiro, and then progressively into Arapuni and beyond.


The actual work was initially messy, time consuming and probably dangerous. However a series of refinements over time lead to the polished operation that it is now.


While the project was on behalf of the wider community ,The permit was issued to the industry organization at the time and is currently held by EECo to do the work. (EECo being the representative company of eel quota owners in the North Island , about half of eel quota  being owned by various IWI or held by TOKM.)


The rational at this time was to prevent the wastage of elvers and provide improved fishing opportunity for the public. Towards  2001 ECNZ was looking to renew, and alter, it's resource consent to operate.  They sought a 35 year term (the maximum) and major changes to the flow regime throughout the system.


As  effected parties, the industry and Maori took a close and concerned interest in the proposal. Of particular concern was the effect of much controlled and reduced flows below karapiro dam . We asserted that these would have a very negative effect on tuna habitats and tuna growth.


We were concerned at the proposed flow regime above Karapiro as well, but as we had little experience in the area , we were not as concerned as we probably should have been. By way of mitigation we sought support to stock the upper hydro lakes with elvers and establish a better fishery. In practice ECNZ would run the attached elver capture equipment at Karapiro  and (EEco) would provide staff for the operation of the traps and transport. To date this agreement has worked well .


Initial stockings had very rapid growth rates and commercial harvests were possible within 2 years of elver release. Over time we have noticed much fluctuation in growth rates, condition and health of eels and also major changes in just about every aspect of the lakes. Because of these changes it is impossible to know what effects what.


But to list these changes, that are of concern, and in no particular order,


  • The change from predominately forested catchment to intensive dairying and the accompanying discharge into the lakes of effluent, fertilizer and sediments.

  • The massive expansion of irrigation uptakes.

  • The very rapid and extensive water level changes , particularly below Karapiro and above the Mihi bridge in lake Ohakuri.

  • The prevalence of algal blooms , especially the whole of lake system blooms such as we saw with the red bloom 3 years ago.

  • The sudden demise of koura, and an apparent decline in snail.

  • The change from predominately the aquatic plant Lagorosiphon to Hornwort and the control measures being implemented to deal with this.

  • The absence of any formal useful monitoring of aquatic organism's  by any relevant authority. Not to suggest that any of this is abnormal, waterways throughout NZ are under immense stress through nutrient overload and water abstraction. 


Clearly the lake system has changed, and will continue to change into the foreseeable future. Maintaining freshwater fisheries of any sort in the current climate will be an immense challenge .


The success of the Karapiro transfer program triggered similar program's throughout the country to the point that they are established practice on nearly all power stations throughout the country.


In spite of these adversities we remain confident that the transfers are delivering benefits to employment and general fishing opportunity.


**Note**   ECNZ became Mighty River Power, which has just recently become Mercury Energy.

Enhancement basically means to make something better.  In this context, we refer to enhancement as:  the capture of elvers at a dam face and release into waters above the dam or elsewhere where they might thrive.  We also mean it to assist any migrating female back down below the lowest barrier/dam to allow her to join the ‘heke’ (migration) to her spawning grounds somewhere west of Tonga.

Between two and three years after glass eels invade the mouth of the Waikato they reach the bottom of the Karapiro Dam. 


In early summer, they explore for ways to continue their ascent up the river.  All avenues are explored.  Wherever water flows are manageable these small elvers will try to climb the concrete.  Unfortunately, if unaided, nearly all die of dessication, exhaustion or predation! 

In 1991, EECo, along with Tainui and Mighty River Power decided to embark on a project to trap them at the bottom of the dam and then release calculated quantities above all the dams in the system and any other local waters with restricted recruitment. 


Permits to trap and transfer under the Fisheries and Conservation Acts were eventually issued after considerable scrutiny by officials.  Conditions to the permits restricted the numbers which could be released in each waterway and required detailed reporting protocols which assist the Ministry of Primary Industry gauge annual levels of recruitment for both longfin and shortfin eels. 

karapiro elver transfer

The program is complex with sophisticated equipment in difficult operating environment.  A safety program is mandatory and taken extremely seriously.

The trap comprises an easily climbed ramp with running water to attract the elvers. 


 Karapiro Dam is massive and the trap will only be successful when positioned where a high proportion of elvers will find it.  The elvers climb the ramp and fall into a holding cage. 


EECO monitors the trap every couple of days during the season, reports the catch and takes samples for NIWA to analyse for species and age amongst other parameters. 

karapiro elver transfer

When sufficient elvers have been collected EECO transfers them to a live fish vehicle and releases permitted quantities in the lakes above each of the upper Waikato hydro dams.


The Project has been a great success!


Approximately 29t of eels (c.25% of the QMA21 fishery) are now caught in the enhanced areas each year. 

The programme has re-opened customary opportunities which have been severely curtailed since the dams were built.


Some Interesting questions & lessons learned

  • Initial growth rates in Lake Arapuni were extremely high when there were few eels and plenty of feed.

  • Growth rates slowed down markedly as the eel population increased.

  • We had in the vicinity of a 4% survival rate which indicates that we were probably overstocking these waters.

  • What to do with the excess elvers?

  • Recent trials in nearby farm ponds replicated the original high growth rates and indicate opportunities for both customary and commercial fishers

  • Eel abundance is heavily restricted by feed availability

  • Many female eels released in the early days are now large enough to mature and join the heke.  Unfortunately, unless they go down over the dam when water is being ’spilled’ they are killed in the turbines.  Niwa and Mercury Energy are trying to find ways to prevent large migrants entering the penstocks.  EECo have successfully lobbied MPI to change the regulations to allow fishers to hold and release these future mothers below Karapiro giving them a clear course for the coast.


Recruitment is now monitored across a number of sites in the North Island including Karapiro, Matahina, Piripaua (Waikaremoana), Wairere Dam and Patea Dam. 


Opportunities to enhance eel stocks are certainly not limited to the Waikato.

A history of eel passage up the Waikato River

The original idea to transfer elvers at Karapiro has quite a long history. Prior to any dams few eels could penetrate past the Arapuni gorge falls in any great numbers.


The first power station was built at Hora Hora in 1913 (below the gorge) and is unlikely to have had any effect on fish passage. The first dam was Arapuni in 1929, which due to the nature of it's spillway allowed elvers to climb past the old gorge and much more easily enter the upper river. Certainly eels were present and being fished in the lakes at least as far up as Ohakuri when elver transfers commenced.


In  1940 control gates were installed at Taupo to regulate flow down the river.

Karapiro dam (the bottom) dam was completed in 1947. As part of it's construction conditions it was designed to " prevent elvers entering the lake". This provision was in the belief that eels would ruin  the trout fishery in the lake.


Following the 2nd world war the remaining 6 dams were built as part of an overall expansion plan with Aratiatia being the last to be  completed in 1964.


Prior to elver transfers, Eels were being  commercially fished as far up as Lake Waipapa, but a lack of recruitment was clearly an issue.

karapiro dam