The Good the Bad & the Ugly

The Good the Bad & the Ugly

I read about the Waikato River Authority (WRA) releasing 76,000 elvers (young Tuna) into the Waikato River at Lake Karapiro during the recent Waka Ama Sprint Nationals 2026.

On reading this press release from the WRA I immediately thought to myself; what is to be achieved by this action? The reason for this thought is set out in the title of this article; the good the bad & the ugly!

The Good:

It’s great to see the WRA taking action to boost the stocks of the Tuna (eels) in the Lake and the Waikato River.

Doing this in such a public manner at the Waka Ama Nationals and gaining some serious publicity for their Kaitiaki status in relation to the river.

In doing so ensuring the next generation see the need for enhancement of the fauna in the river by regularly releasing more elvers into the river.

The Bad:  

My first thoughts were, did anyone take into account the effects of Koi Carp on the elvers that were being released into the river.

My reasons for this thought are as follows.

Estimates for koi carp in the Waikato catchment vary significantly depending on the specific water body and the study's timeframe. Recent figures from late 2025 suggest a total biomass of 500,000 to 750,000 tonnes within Waikato waterways, with the highest concentrations found in the lower Waikato catchment.

Specific population and biomass breakdowns include:

• Total Catchment Biomass: Estimated at 500,000–750,000 tonnes. Based on an average weight of 3kg per fish, this equates to somewhere between 16,666,666 and 25,000,000 individual fish in the catchment.
• Lower Waikato Catchment: Koi carp are estimated to occupy 80% of the total fish biomass in this region.
• Lake Waikare: Estimates place approximately 8,000 tonnes of koi carp in this lake alone. Based on an average weight of 3kg per fish, this equates to roughly 2,666,666 individual fish in Lake Waikare.
• Waikato River (Karapiro to Estuary): Preliminary research indicated at least 65,000 koi (roughly 70 tonnes) in the main river channel and side channels, though researchers consider this a very conservative underestimate.
• Lake Waahi: Estimated to hold approximately 32,000 (75 tonnes).

A simple Google information request (Do Koi Carp eat juvenile tuna elvers) came back with the following information:

Yes, koi carp (Cyprinus carpio) are opportunistic omnivores and will eat juvenile tuna (elvers) if they encounter them.

Feeding Behavior: Koi are bottom-feeding, omnivorous fish that "vacuum" up organic matter, including small fish, fish eggs, insects, and larvae.

Impact on Native Species: In New Zealand, where tuna (native freshwater eels) are a valued species, koi carp are considered a major pest that threatens indigenous biodiversity by consuming eggs and larvae and destroying habitats.

Context: While they are not active hunters, their feeding style involves eating any small, digestible organism, making them a threat to small, slow-moving, or newly migrated juvenile fish.

Therefore, in waterways where both species coexist, such as in the Waikato River catchment, koi carp will consume elvers and, unless or until, something is done to eradicate the Koi Carp from the waterways the release of more elvers is not going to solve the problem of the reducing numbers of Tuna in the waterways in any meaningful way.

The Ugly:

When you take into account the numbers of individual fish estimated to be in the catchment overall and then add the reproductive capacity of the Koi Carp the results become very ugly very quickly.

An average fish weighs 3 kg. Females produce 100 000 eggs per kg of body weight. A typical female can produce 300 000 eggs annually (or more if they spawn¹¹ more than once). Koi carp spawn throughout the summer. As they gather for spawning¹² or feeding in the shallow margins of the river, Koi biomass can reach 4000 kg/ha.

Whilst Koi Carp are not active hunters of the elvers, they will eat them whenever the elvers come into contact with the Koi Carp.

Given the estimated numbers of between 16 & 25 million Koi Carp in the Waikato catchment and the number of elvers released at 76,000, the chances of them surviving in great enough numbers to make a difference in the overall health of the Tuna biomass is not good.

Also taking into account the rate of reproduction of the Koi Carp and their lack of any real predators in the NZ it is obvious that the problem of Koi Carp is one that is exponentially growing at a rate that is much greater than we can possibly combat by releasing more elvers, until action is taken to control and eradicate the Koi Carp.

So in Summary I would give the WRA a thumbs up for trying to do something to improve the Tuna biomass in the catchment but I have to also give them a much greater thumbs down for failing to take any actions up to the present time, to either control or eradicate the Koi Carp.

In my opinion all they have practically achieved with this release of elvers is to improve the feedstock for the Koi Carp.