What does it look like?
Tench grow to a large size in New Zealand - around two kilograms. They are usually olive green-bronze in colour, with red eyes, two barbels, large soft-rayed fins and copious mucous. Adults are approximately 30-40cm long (sometimes up to 70cm) and the lifespan is around five years. Male and female fish look slightly different - males have longer and fatter pelvic fins than females.
Juveniles predominantly feed on zooplankton. Adults are primarily bottom feeders, preferring small molluscs when available, but can survive solely on zooplankton when there are no alternative food sources. Tench are predominantly nocturnal foragers and are a warm-water species, becoming less active during winter.
Tench prefer shallow regions of warm, still or slow-moving freshwater bodies with soft substrates of mud, silt, sand and preferably some submerged vegetation. They tolerate very low oxygen levels, high turbidity, brackish (salty) water, and a wide pH range.
Why is it a problem?
The biology of Tench in New Zealand has not been studied but is probably similar to the same species in their native European waters. Tench generally live in still or slow-flowing waters and are carnivorous, feeding mainly on crustaceans, molluscs and insect larvae. This means they have the potential to significantly alter native freshwater communities and have few predators in New Zealand. Tench may compete with common bullies as both are bottom dwelling, but no substantiating evidence of any effects is available.
Tench spawn in shallow water, broadcasting eggs over substrate. Multiple spawning events (batches) can occur within one season. Warmer temperatures favour earlier sexual maturation and higher fecundity.
More information
Although imported to New Zealand as a sports fish in 1867, there has been little angler interest in tench until recently.
A golden variety of tench - which is bright yellow-orange - is thought to be present in some small lakes and ponds near Auckland. This variety of tench was illegally imported in around 1980.
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Notify Council
If you think you've found this pest, please get in touch with our Biosecurity Team at biosecurity@hbrc.govt.nz or call us on 0800 108 838.
Management Programme
Unwanted Organism
An unwanted organism is any organism that's capable of causing harm to natural or physical resources (like forests and waterways) or human health. A number of introduced pests in New Zealand are classed as unwanted.
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Sections 52 and 53 of the Biosecurity Act (1993)
No person shall knowingly communicate, cause to be communicated, release, or cause to be released, or otherwise spread any pest or unwanted organism, in accordance with Sections 52 and 53 of the Biosecurity Act 1993
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