The Australian grayling (Prototroctes maraena) is a primarily freshwater fish found in coastal rivers in Tasmania and south-eastern Australia. In past decades it has also been known as the âcucumber mulletâ or âcucumber herring,â for its cucumber-like odor.
Description and Diet
The Australian grayling is a streamlined fish with a long and slender body and small conical head. Colouration is usually silver on the flanks and dusky olive on the back, overlain with a gold sheen.
Australian grayling commonly live for 2â"3 years and reach around 20Â cm in length, although rare individuals have been recorded up to 5 years in age and 33Â cm in length. The fish has an omnivorous diet, feeding upon algae, shrimp, and small insects. They have specially adapted teeth and a long gut to help with the digestion of algae.
Australian grayling spawn in the freshwater reaches of coastal rivers. Spawning is thought to occur in late autumn or early winter. McDowall (1996) reports that egg counts range from 25,000 to 67,000 in females 170â"200Â mm long, and that the small (~1Â mm) demersal eggs probably settle among gravel and cobble in the river bed before hatching. Hatched larvae are washed out to sea. Australian grayling juveniles return to the freshwater reaches of rivers after roughly 6 months at sea and spend the rest of their lives in river habitats.
Before the introduction of exotic fish species including the Eastern mosquitofish and trout to Australian waterways the Australian grayling and spotted galaxias were keenly fished by recreational anglers using fly-fishing gear. The species was appreciated for its willingness to take wet and dry flies, its excellent fighting ability on very light tackle, and its relatively large size. However, due to declining numbers the fish is now protected under the Environment Protection and Biodiversity Conservation Act 1999. Heavy penalties apply for taking any of the fish.
Historical Declines and Current Threats
Australian grayling suffered massive initial declines in 1869â"70 by very large, unexplained fish kills (Saville-Kent, 1888). Descriptions of grayling killed in this event of being covered in "cottony growths" suggests a fungal pathogen, possibly marking the arrival of exotic Saprolegnia fungus into Australian freshwater habitats thanks to the reckless importation of exotic salmonid species.
Saville-Kent then went on to consider the apparent epidemic some 17â"18 years previously which had caused the demise of the Australian grayling Prototroctes maraena. The grayling were said to "have been seen floating down the rivers in thousands, covered more or less extensively with a cottony fungoid growth. So virulent and exhaustive was this epidemic that many, more especially of the southern rivers, were more or less completely denuded of their stock of this species and have so remained up to the present date". Saville-Kent posed the questions of how, when and where the epidemic originated and whether at the time there were any abnormal conditions associated with the rivers carrying the infected fish. He went on to say: "The approximate date of the appearance of this epidemic would appear to be about the year 1869 or 1870, periods it may be remarked of great activity in association with the distribution of the fry of the newly acclimatised Salmonidae in the rivers of this colony. Is it possible ...that the fungus, ââSaprolegniaââ, was hitherto unknown to Tasmania and was introduced with the ova of these Salmonidae, or more probably in the moss wherein they were packed? Under such conditions the germs or spores, like the microbes of measles or smallpox, arriving on a virgin and congenial soil, might be expected to spread with devastating virulence among the aboriginal inhabitants."
Today Australian grayling are threatened by a number of things. Dams and weirs block migration and also block floods and reduce base flows, both of which are important for habitat maintenance and for spawning and movement of grayling larvae and juveniles to and from the sea. Irresponsible forestry and farming practices degrade and fragment river environments through siltation and other effects. Exotic trout species threaten grayling through predation and competition. Scientific studies have shown native fish species similar in habitat and lifestyle such as spotted galaxias are severely depressed in number in rivers inhabited by exotic trout species (Ault & White, 1994), and are forced into sub-optimal feeding locations, feeding times and diets by aggressive competition from exotic trout species (McDowall, 2006). A chronic lack of exotic-trout-free habitat reserved for galaxias species and other native fish species in south-eastern Australia generally is a major concern.
Australian grayling are listed as a vulnerable species under Australiaâs Environment Protection and Biodiversity Conservation Act 1999 and as Near Threatened under the IUCN Red List. There are now a number of conservation measures focused on conserving the fish.
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- Cadwallader, P.L. (1996) Overview of the Impacts of Introduced Salmonids on Australian Native Fauna. Australia Nature Conservation Agency, Canberra.
- McDowall, R.M. (1976) Fishes of the Family Protroctidae (Salmoniformes). Australian Journal of Freshwater and Marine Research 27: 641â"659.
- McDowall, R.M. (ed.) (1996) Freshwater Fishes of South-Eastern Australia. Reed Books, Sydney.
- McDowall, R.M. (2006) Crying wolf, crying foul, or crying shame: alien salmonids and a biodiversity crisis in the southern cool-temperate galaxioid fishes? Rev Fish Biol Fisheries 16: 233â"422.
- Saville-Kent, W, 1888. On the acclimatisation of the salmon (Salmo salar) [and exotic trout species] in Tasmanian waters, and upon the reported disease at the breeding establishment on the River Plenty. Papers and Proceedings of the Royal Society of Tasmania 1887: 54â"66.
- Wager (1996). Prototroctes maraena. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 10 May 2006. Listed as Vulnerable (VU A1c v2.3)
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- Australian Inland Fisheries Service