Resource Library

Freshwater fish

DATED: 03 Feb 2014

Version: Aquatic Pests - Freshwater fish (Second edition)

Freshwater fish


Brown bullhead catfish

Ameiurus nebulosus

Brown bullhead catfish (Photo: Michael Kesl).Brown bullhead catfish (Ameiurus nebulosus) Photo: Michael Kesl.

What do they look like?

Brown bullhead catfish are dark brown to olive green in colour with paler sides and bellies. They have eight distinctive barbels around their mouth, small eyes and smooth skin. They also have a toxic spine at the leading edge of their dorsal and pectoral fins. Adult fish grow to 20-30cm long.

Why are they a problem?

Brown bullhead catfish are carnivorous, preying on many native species and competing with others – like freshwater crayfish (koura) – for food. They use their barbels to locate insects, crustaceans, molluscs and small fish.

They also stir up mud, reducing water quality for other animals and plants. This hardy fish, known to be present in a number of Northland waterways, can survive in poor water quality and out of water for a long time.

Brown bullhead catfish are widespread in Northland but are not known to be present in any of the region's high-value waterways.

Koi carp

Cyprinus carpio

Koi carp (Photo: DOC).Koi carp (Cyprinus carpio) Photo: DOC.

What do they look like?

Koi carp look a bit like big, motley-coloured goldfish with whisker-like feelers (barbels) at the corner of their mouths. They commonly grow to over five kilograms and more than 60cm long. Their colour varies, but is typically a calico pattern of black, red and orange-gold with white blotches.

Koi carp prefer still waters in lakes or backwaters in rivers and are very tolerant of poor water quality.

Why are they a problem?

This pest eats nearly anything it comes across including insects, spawn and juveniles of other fish species, and a wide range of aquatic plants. Koi carp can significantly reduce water quality as they feed along the bottom like a vacuum cleaner, stirring up sediment and expelling what they don't want. The muddied waters they create can destroy native habitats and make water unsuitable for swimming and livestock water supply.


Scardinius erythrophthalmus

Rudd (Photo: U.S. Geological Survey).Rudd (Scardinius erythrophthalmus) Photo: U.S. Geological Survey.

What do they look like?

Rudd are a stocky, deep-bodied fish with large scales that range in colour from silver to copper-orange. They are darker on their backs than on their bellies and their fins are usually bright reddish-orange. Widespread around Northland, this pest fish usually grows to 25cm. Rudd look similar to koi carp but without the whisker-like feelers.

Why are they a problem?

Rudd are prolific breeders producing over 50,000 eggs per kilogram of fish weight. In high densities rudd can endanger native aquatic plants, compete with native fish and invertebrates for food sources and potentially degrade water quality by stirring up bottom sediments. Juveniles are carnivorous, feeding mainly on invertebrates but the adult diet consists mainly of aquatic plants.


Leuciscus idus

Also known as: ide

Orfe (Photo: Piet Spaans).Orfe (Leuciscus idus) also known as: ide - Photo: Piet Spaans.

What do they look like?

New Zealand's orfe is an ornamental strain called the golden orfe. It looks very similar to rudd, but with smaller scales and fins that are more orange than red in colour.

This pest fish was illegally introduced into New Zealand in the 1980s for fishing. The fish was released into one Northland lake, but doesn't seem to have established. However, the current status of the wild population is unknown and it is likely that some orfe persist in the wild.

Orfe prefer slow-flowing water.

Why are they a problem?

Orfe can tolerate high levels of salinity meaning they may be able to colonise estuaries which are critical habitats for many of our native fish species. They are prolific breeders.

This pest generally eats aquatic invertebrates and snails, but large ones may also feed on other fish and vegetation.


Perca fluviatilis

Perch (Photo: Piet Spaans).Perch (Perca fluviatilis) Photo: Piet Spaans.

What do they look like?

Perch have two dorsal fins with sharp spines on the front one. They are olive-coloured with darker bands on their sides, but their underside fins and the bottom of their tail are a bright orange-red. Adults generally weigh about 2 kilograms.

Perch prefer to live in slow-flowing or still waters.

Why are they a problem?

Perch are the worst fish pest in New Zealand. They have the potential to significantly alter native freshwater communities through predation and competition with native fish species.

This pest is carnivorous and eats insect larvae and other fish. Perch were released in New Zealand in the 1870s as an angling fish and are not common in Northland.


Tinca tinca

Tench (Photo: Steffen Zienert, U.S. Geological Survey).Tench (Tinca tinca) Photo: Steffen Zienert, U.S. Geological Survey.


What do they look like?

A distinctive feature of tench is their bright orange eyes. Tench grow to a large size in New Zealand and fish over two kilograms are not uncommon. They are olive green in colour with very fine scales covering their bodies. Their fins tend to be thick and fleshy and they have a single, small feeler (barbel) at each corner of the mouth.

Tench generally only live in still or slow-flowing water.

Why are they a problem?

Tench have the potential to significantly alter native freshwater communities through predation and competition with native fish.

This pest feeds on insect larvae, crustaceans and molluscs.

While not common or widespread in Northland, tench are known to be in least two lakes in the region.

Wild goldfish

Carassius auratus

Wild goldfish (Photo: Stephen Moore, Landcare Research).Wild goldfish (Carassius auratus) Photo: Stephen Moore, Landcare Research.


What do they look like?

Wild goldfish lack the bright colours and feathery fins of their domestic relatives. They are bronze to gold in colour with large scales, and grow to 20cm long. They look similar to koi carp but without the whisker-like feelers (barbels) around their mouths.

Wild goldfish prefer in still waters (ponds and lakes), but also inhabit slow flowing rivers.

Why are they a problem?

Wild goldfish degrade freshwater environments by out-competing native fish species and other aquatic life.

Wild goldfish are thought to be widespread in Northland. They are prolific breeders that have the ability to breed with koi carp.

Recent research shows that wild goldfish can contribute to the intensity and frequency of blue-green algae blooms.


Gambusia affinis

Also known as: mosquito fish

Gambusia (Photo: NOZO).Gambusia (Gambusia affinis) also known as: mosquito fish - Photo: NOZO.

What do they look like?

Gambusia are small, silver-coloured fish with dark-edged fins, rounded tail fins and a single, high, rounded dorsal fin. Adults grow to 6cm long. They are widespread in Northland - having been introduced into many countries due to the misconception they could help control mosquitoes by eating the larvae.

Why are they a problem?

Though small, gambusia are very aggressive and frequently attack native fish, nipping at their eyes and fins. They also eat fish eggs. Gambusia mature quickly and breed rapidly, so populations quickly expand to outnumber other species.


Phallocerus caudimaculatus

Caudo (Photo: Mark Maddern).Caudo (Phallocerus caudimaculatus) Photo: Mark Maddern.

What do they look like?

Caudo are a popular aquarium fish but were recently discovered in the wild. They are similar in size and shape to their relatives, gambusia, but look more yellowish. Males are distinctively speckled with irregular black blotches. Caudo live in still ponds and prefer habitats with dense vegetation. In Northland they have been found in stock water troughs.

Why are they a problem?

Little is known about the impacts of caudo in the wild but, given their similarities to gambusia, they are likely to be a real threat. They may affect native fish populations through predation, competition for food and habitat, aggression and the introduction of parasites.