Koi carp are big, bold and beautiful. But looks can be deceptive; these docile but destructive fish have become the possums of Waikato’s waterways.
Koi are an ornamental strain of common carp (Cyprinus carpio), native to Asia and Europe. They were accidentally introduced to New Zealand in the 1960s as part of a goldfish consignment. Because they are spectacular fish, many fish fanciers desire them for their ornamental ponds.
Koi prefer still waters in lakes, or river backwaters and margins, and have become widespread in Auckland and Waikato. They are also spreading into Northland and have been found in isolated areas around Wanganui, Hawke’s Bay and Wellington.
Drs Brendan Hicks and Nick Ling from the University of Waikato’s Centre for Biodiversity and Ecology Research, in the Department of Biological Sciences, are heading a major study looking into the ecology and control of koi in the Waikato region. They’re also attempting to discover if there are weak points in the fishes’ biology that could be exploited to control or eradicate them.
The Foundation of Research, Science and Technology has recently granted the University of Waikato $10 million over 10 years to study pest fish and harmful algal blooms.
Koi feed by grubbing up lake-and river-beds and ripping out water-weeds, which creates turbid, nutrient-rich water, so increasing eutrophication and oxygen depletion, as well as reducing light and clogging the gills of other aquatic animals. Many once healthy Waikato lakes have been seriously degraded by the fishes’ activities.
“When koi feed, their tails protrude from the water because they’re head down, tail-up, rooting up the bottom like a pig. The mess a pig makes in a paddock is the mess koi make underwater,” says Hicks. “We assume they feed on small invertebrates, but they’re omnivorous and also feed on plant material and detritus. Koi cause quadruple problems of plant removal, sediment release into the water, consumption of much of the biomass and excretion of nutrients into the water, which increases algae growth. As well as increasing the water turbidity, the rooted up plants no longer hold the lake or river-bed together.”
Hicks will investigate the ecology of the problem, whereas Ling is concentrating on the koi’s physiology and natural history. He comments: “The lake fish seem to be in a poorer condition than the river fish, which is probably because they inhabit a more limited resource. Their population is getting so large that they’re ruining their environment.”
Australian studies have set a threshold koi carp population density of 450 kg/ha, but the Waikato study has discovered some sites with over 1600 kg/ha. Recent surveys undertaken for Environment Waikato have shown that koi may now make up 80-90 per cent of the biomass of some waterways. Koi average 2-4 kg at 4-6 years of age.
Koi survive in habitats that completely ice over in winter, and New Zealand’s highest temperatures are insufficient to kill them or stop them from spawning. Their substantial size also ensures that they aren’t subject to significant predation.
The fish produce 30,000-800,000 eggs per kilo bodymass (100,000 on average), and some large females carry several million. The survival rate of eggs and infant fish is probably less than 0.1 per cent, but sheer volume is the problem. Koi spawn in spring when the water reaches 15-16° C, and can continue spawning throughout summer.
“Recent studies have shown some females with three batches of eggs at different stages of development in the ovaries at one time,” says Hicks.
A 4.5 m electro-fishing boat developed by Hicks and the Department of Biological Sciences technical manager, Dudley Bell, is now used to capture koi for tagging. This craft has many advantages over conventional fish-catching techniques such as netting. The electrical field reaches bottom and mid-water dwellers and fish of all sizes, whereas netting usually takes fish from only one location and across a narrower size range.
The boat is designed with a shallow “V” hull and auto-tilt motor so it can get into the shallow weedy margins (as little as 0.2 m deep) where the carp tend to congregate. Over 1300 fish have now been tagged.
Tagged fish are recovered by the scientists themselves as well as by bowfishers, coarse fishermen and commercial eelers. Retrieval of tags, or preferably entire fish, allows growth and movement patterns to he studied.
Koi colonise the Waikato River’s broad, shelved margins. Willows grown to stop river-bank erosion provide ideal shaded habitat, while their overhanging branches are a hindrance to reaching the fish with the boat.
So far, about 30 tagged fish have been recaptured, which is a good rate for such a large population. Recaptured fish are weighed and measured, and four scales are removed for age determination. Most don’t seem to have moved more than a kilometre or two from where they were tagged, yet at some sites where fish were tagged there have been no recaptures.
Koi are a prime target for the local bowhunting fraternity. Huntly’s World Koi Classic bowfishing competition provides the scientists with a golden opportunity to sample a vast number of fish. It also highlights the number of pest fish inhabiting the waterways. In the 2004 competition, 8.6 t of koi (3500 fish) were shot in two days.
New Zealand Bowhunters Society president and Huntly local Alan Metcalfe indicated just how many koi inhabit some lakes. “One method of hunting koi is to stay on the lake-bank and watch a clear spot among the weeds. A friend and I shot 100 carp in one day using that technique. They just kept going past!”
Because koi live in dirty water, they can’t use visual signals like other fish and so rely on pheromone signals to congregate. Some of these chemical attractants are thought to reside in the bile. The biologists hope to be able to isolate the pheromones and use them to cause the fish to congregate and thus increase the capture rate.
Other pest-fish species are causing problems in the Waikato region too. Tench, rudd and perch are having an impact on some waterways. These are classed as sport fish, and some people have been illegally transporting them to new locations.
“Once those other species become established, a licensed fisherman is legally allowed to fish for them, whereas koi are classified as noxious fish and anyone can kill them,” said Hicks.
One unexpected discovery has been the vast numbers of goldfish (Carassius auratus) in koi habitats. On the other hand, relatively few catfish (Ameiurus nebulosus) have been found, and it’s thought that koi are influencing their numbers, as both are bottom feeders. There’s also anecdotal evidence from commercial eelers of declining catfish numbers.
“If koi ever reached the Rotorua or Taupo lakes, it would devastate our internationally renowned trout fisheries,” says Hicks. “People must realise that koi are a serious threat to our waterway’s biodiversity.
They must never be introduced to other waterways. Any recreationally-caught fish must be killed and not released.”
In Australia, the Commonwealth Scientific and Industrial Research Organisation is looking at new ways to eradicate carp and has developed a system of genetic modification called daughterless technology. The GM fish produce only male offspring. These inherit the modified gene responsible and in turn pass it on whenever they mate, as do their (male-only) offspring—and so on. Modelling suggests a wild population could be driven into extinction in 10 to 15 years.
However, public dissaffection with GM technology is likely to preclude such an approach being taken in New Zealand in the foreseeable future.