Most people are unaware our unique native freshwater fishes. They are mostly small and like to remain inconspicuous to protect themselves from predators. You are much more likely to see an introduced fish, such as gambusia or goldfish in your local stream, than to see a native fish. But that doesn’t mean that the natives are not there. You just have to look a bit harder.
There are 11 species of native freshwater fish in the south-west of Western Australia, and nine of these are found nowhere else in the world (the other two species are also found in eastern Australia). Around Perth, the species we are most likely to see are freshwater cobbler (Tandanus bostocki), western minnow (Galaxias occidentalis), western pygmy perch (Nannoperca vittata) and nightfish (Bostockia porosa).
Freshwater cobbler (Tandanus bostocki)
This is the largest native species in our part of the world. Cobbler are a type of catfish. They can grow to 500 mm in length, with an eel-like tail and four pairs of fleshy projections, called barbels, around the mouth. They also have a sharp, venomous spine on their back – so be careful if you try to handle them.
Cobblers are light brown in colour, with a darker head and back, and distinctive mottling over the body.
Cobblers are active at night. They prefer deeper water and like to stay close to the bottom of the stream, where they feed on invertebrates such as freshwater crayfish and mussels. They are usually seen singly, although juveniles sometimes occur in shoals.
Although they will undertake small upstream migrations, cobbler have quite a small home range and will usually spend most of their time in the same section of the stream, often in deep pools that maintain water over the dry summer/autumn period.
Freshwater cobblers are not listed as threatened, but their range does appear to be declining. They are the only native freshwater species in the south-west that are fished for sport or food.
Video supplied by Fish Health Unit, Murdoch University.
Nightfish – Bostockia porosa
Nightfish are related to Western Pygmy perch and have a similar shape, but they are larger (up to 170 mm), with conspicuous pores on the head and a large mouth.
In colour, they are mottled olive brown to dark brown, with a paler belly.
Nightfish prefer still or slow-moving waters. Adults are solitary and nocturnal, hiding amongst rocks and woody debris during the day. They are best seen using a torch at night. Rocky pools are ideal places to search. They are ambush predators, waiting on the bottom to snap up insects, crustaceans, snails and small fishes that venture too close.
Although rarely seen, nightfish are reasonably common and widespread. However, they do seem to be declining in catchments that are salinized or where clearing has reduced the instream woody debris that they use for shelter.
Western Pygmy Perch
Western Pygmy Perch (Nannoperca vittata)
Western pygmy perch are small fish, with a maximum length of 80 mm, and perch-like in shape (although they are not closely related to the perches of Europe and North America), with a small mouth.
They have a mottled, golden-brown coloration. During the breeding season (July to November) males become brilliantly coloured, with a mottled golden pattern on the sides, red-orange on the belly and dark fins, while females develop a blueish tinge.
Pygmy perch can tolerate a range of water conditions, both flowing and static and are reasonably tolerant of saline conditions. They are typically found in aquatic vegetation on the margins of streams or lakes. They have a varied diet of benthic crustaceans and terrestrial insects, and are especially voracious consumers of mosquito larvae. Studies have found that pygmy perch are in fact much better at controlling mosquito larvae than are gambusia, which were originally introduced for that purpose.
Western pygmy perch are one of the most common and widespread native fishes in south-western Australia, although they have declined in upper catchment areas where water is very saline. Because of their hardy nature, ease of breeding and ability to control mosquito larvae, they are often sold in aquarium shops for stocking in outdoor ponds.
Western Minnow – Galaxias occidentalis
This is one species of native fish that is usually easy to spot if it is present. Minnows are slender, torpedo-shaped fish, usually around 100 mm in length, although they can grow up to 200 mm.
Their colour ranges from olive green to light brown, with a series of darker bars on the upper sides.
Western minnows are active, fast swimming fish. They are often seen in shoals, swimming just above the bottom. They like well-oxygenated water are often found in fast-flowing streams near rapids and waterfalls. They eat small crustaceans and terrestrial insects that fall into the water.
Western minnows are widespread and reasonably common, although they have largely disappeared from the upper reaches of salinized rivers.
Video: Fish Health Unit, Murdoch.
In Western Australia feral fishes are found mainly in waterways in the vicinity of major urban areas. These introduced freshwater fishes include Goldfish (Carassius auratus); Koi Carp (Cyprinus carpio); Rainbow Trout (Oncorhynchus mykiss); Brown Trout (Salmo trutta); Eastern Mosquitofish (Gambusia holbrooki), One-spot Livebearer (Phalloceros caudimaculatus); Guppy (Poecilia reticulata); Swordtail (Xiphophorus helleri); Redfin Perch (Perca fluviatilis); Mozambique Mouthbrooder (Oreochromis mossambicus); and Eastern Australian Silver Perch (Bidyanus bidyanus).
Feral fishes are a threat to unique native freshwater fishes because they can tolerate degraded habitats, are highly adaptable and reproduce readily. Feral fishes are: predators of native freshwater fishes; compete with native fishes for food decrease water quality; and introduce parasitic diseases, such as anchor worm. They are a significant threat to native freshwater fishes in the south-west of Western Australia, where 82% of fishes are unique local species and 55% of species are endangered.
Eastern Mosquitofish (Gambusia holbrooki)
The Eastern Mosquitofish (Gambusia holbrooki) was introduced into Western Australia to control mosquitos in the 1930’s and is now extremely abundant in the south-west and southern Pilbara region.1 The species can survive in a range of habitats, including very saline water, which has allowed Eastern Mosquitofish to dominate rivers in cleared catchments that have become saline, such as the Blackwood River catchment.1 The Eastern Mosquitofish has not been found in most of the Kimberley and Pilbara regions yet as it has not been released into these regions.
The Eastern Mosquitofish is very aggressive and causes severe fin damage and death of native fishes. Where lakes and rivers lack cover, there are few native fishes if the mosquitofish is present, compared to habitats that have cover.1
Goldfish (Carassius auratus), one of the more popular species of ornamental fish, are now relatively widespread throughout the world. Feral Goldfish prey on the eggs, larvae and adults of native freshwater fishes and compete for food and space. Goldfish escape predation from a young age as they grow much bigger than nearly all native freshwater fishes. Goldfish stir up sediment and deplete aquatic vegetation when they feed on the bottom of waterways, which reduces habitat and spawning sites for native fishes. Goldfish introduce parasites and other serious diseases to native fishes. They stimulate cyanobacteria growth in nutrient enriched environments, which causes algal blooms. It is likely that Goldfish will become more established in the future due to deliberate release into waterways.
The Guppy (Poecilia reticulata), is an ornamental fish that loses its ornamental tail in the wild so it looks more like a Swordtail rather than a native fish. The guppy has only been found in one site in the Pilbara.
The Swordtail (Xiphophorus helleri) is a very popular ornamental fish and has been promoted for aquaculture in Western Australia. The Swordtail flourishes in habitats that have been modified by humans and is often found near urban areas where the likelihood of being deliberately released into waterways is increased. The species is invasive because of the ability to tolerate a wide variety of environmental conditions, high fertility and fast growth rates. The male Swordtail is territorial and spends much of its time aggressively fighting with other males and other species. Swordtails out-compete native fishes for food and habitat and predate on their eggs and juveniles. These negative impacts increase particularly when the species occurs in high numbers, or when it lives in habitats that have other introduced feral species, such as the Eastern Mosquitofish. Endemic fishes with restricted localised distributions are particularly at risk when swordtails and other introduced fishes are present.
In Western Australia, the Swordtail has only been found in the Irwin River near Dongara. However, the large population centres in the south-west of Western Australia are areas where the Swordtail is most likely to be released, and areas where human modified aquatic habitats occur that are successfully inhabited by other feral fishes. As the Swordtail can eat a wide range of food, produce live young in large numbers in a short time, is not constrained by environmental conditions and can co-exist with the Eastern Mosquitofish, it is considered a pest by researchers.4
Redfin Perch (Perca fluviatilis) were introduced by anglers into Albany in Western Australia for recreational fishing in the 1890s. These invasive fish have spread rapidly into dams and waterways in the south-west of Western Australia. Redfin Perch are mature at a young age, grow rapidly, can tolerate a wide range of environmental conditions and have no predators. These fish consume marron, gilgies, frogs, insects and native fishes. Researchers consider that Redfin Perch are responsible for the extinction of the mud minnow from Big Brook Dam in Pemberton and population decline of marron.5 Redfin Perch are illegally translocated by anglers, which is considered to be the greatest threat, after habitat degradation, to endemic fishes and crustaceans in the south-west of Western Australia. 5
The Mozambique Mouthbrooder (Oreochromis mossambicus), or Tilapia, is an ornamental fish that has been found in the Chapman and Gascoyne Rivers, adjacent to Geraldton and Carnarvon in Western Australia. These fish were probably introduced by deliberate release and spread to other waterways during floods. The Mozambique Mouthbrooder is very adaptable, eats a wide variety of food, and can tolerate extreme environmental conditions, such as salty water.2 This gives the Mozambique Mouthbrooder a competitive advantage over native fishes. Researchers have found that where there are high densities of Mozambique Mouthbrooder, there are fewer native fishes.3
The climate in the region where they are currently found is semi-arid and during extended dry periods some rivers become small isolated pools, which increases competition for food and space between the Mozambique Mouthbrooder and native fishes. The male Mozambique Mouthbrooder is aggressive and territorial, particularly during breeding. If large areas of the river are occupied by Mozambique Mouthbrooder nests, then this restricts the movement of native fishes. There is a high probability of further range expansions of this fish in Western Australia due to natural dispersal and human-mediated translocation. The Mozambique Mouthbrooder also has a similar diet to that of native freshwater fishes in the region with restricted distributions. As the range of the Mozambique Mouthbrooder is expanding, it is likely that a greater number of native fishes will be impacted by this invasive feral fish.