What does it look like?
Tench were first introduced to New Zealand in 1867 as a sports fish. They grow to a large size in New Zealand, and fish over 2kg in weight are not uncommon.
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 plus years. Male and female fish look slightly different.
Juveniles predominantly feed on zooplankton. Adults are primarily bottom feeders, preferring small molluscs when available, but can survive solely on zooplankton when alternative food sources are absent. Tench are predominantly nocturnal foragers, and are a warm-water species, becoming less active during winter.
Shallow regions of warm, still or slow-moving freshwater bodies with soft substrates (mud/silt/sand) and preferably some submerged vegetation. They tolerate very low oxygen levels, high turbidity, brackish water and a wide pH range.
Why is it a problem?
Tench feed on insect larvae, crustaceans and molluscs, and have the potential to significantly alter native freshwater communities. They have few predators in New Zealand. May compete with common bullies as both are bottom dwelling, but no substantiating evidence of 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. Recruitment varies between years in response to environmental variables, resulting in strong year classes being apparent.
Please contact the Northland Regional Council if you think you have seen this pest.