LAKES | PONDS | RIVERS
problems and solutions

PROBLEMS Pest Fish


Koi Carp (Cyprinus carpio)
Koi Carp (Cyprinus carpio)
Bullhead Catfish (Ameiurus nebulosus)
Bullhead Catfish (Ameiurus nebulosus)
Gambusia (Gambusia affinis)
Gambusia (Gambusia affinis)

Many of New Zealand's waterways have been invaded by exotic pest fish which are thriving in NZ conditions.

Pest fish such as Koi Carp (Cyprinus carpio) native to Asia and Europe were introduced to New Zealand accidentally in the 1960's as part of a goldfish consignment. Coupled with human intervention introductions have occurred elsewhere which the Department of Conservation believes was for the purposes of coarse fishing and for ornamental purposes in amenity ponds. They feed like a vacuum cleaner stirring up the bottom of ponds, lakes and rivers, muddying the water (depleating oxygen levels) and destroying native plant and fish habitat. Koi carp are opportunistic omnivores, whch means they eat a wide range of food, including insects, fish eggs, juvenile fish of other species and a diverse range of organic matter.

Bullhead Catfish (Ameiurus nebulosus) were mistakenly introduced as Channel Catfish into New Zealand in the 1870s - for the purposes of food. Bullhead Catfish look similar at juvenile ages yet never grow on to become a viable food source. Bullhead Catfish are robust fish with distinctive whisker-like barbels (feelers). They have sharp spines at the front of the pectoral and pelvic fin and grow to around 30 cm in length. Unfortunately they stir up sediment and prey on any native fish and koura (native crayfish) that they can fit in their mouths. Like Koi Carp, all catfish must be killed on capture and not returned to the water alive.

Gambusia (Gambusia affinis) formerly known as mosquito fish were introduced to New Zealand in the 1930's from the Gulf of Mexico in an attempt control mosquito larvae, however they are aggressive and frequently attack native fish, nipping at their eyes and fins - also eating other species fish eggs . Mature females grow to 6 cm and males to 3.5 cm. They mature at six weeks old and are short lived but breed rapidly and repeatedly enabling populations to build up to large numbers very quickly. Females give birth to live young. Consequently only one pregnant female is needed to start a new population. (Department of Conservation Website)

NZWR SOLUTIONS Pest Fish Removal


Netting Koi from Lake Whangapae
Netting Koi from Lake Whangapae

NZWR has developed and employs a wide range of techniques for removing pest fish. Most commonly pest fish removals are achieved through a combination of netting, traps, baiting and electric fishing machines.

Specialised netting designs and techniques have also been developed to catch fish to ensure minimal damage to all species captured. The pest fish species can be separated and the native and other desired species returned to the water way with minimal impact/damage. The mesh sizes used are too large for the majority of native species. The NZWR netting technique is capable of removing up to 90% of the pest fish from the water body.

The second phase of pest fish removal (to remove the remaining 10%) is a pelletised feeding system where the fish are trained over a period of weeks (between 2 – 3 weeks). The fish are concentrated in an area through feeding species specific pelletised feeds. NZWR's current pelletised feeds have been designed to retrieve White Amur. However these formulations are available to capture other fish species such as Koi Carp. The feeding system can potentially incorporate toxicants (piscicides) if required. Automated feeding systems have been developed to train the fish to feed in a concentrated area.

For more information or if you would like to inquire you can call us on 0800 NZ WATERWAYS (0800 699 283).