Anguilla australis

The short-finned eel (Anguilla australis), also known as the shortfin eel, is one of the 15 species of eel in the family Anguillidae. It is native to the lakes, dams and coastal rivers of south-eastern Australia, New Zealand, and much of the South Pacific, including New Caledonia, Norfolk Island, Lord Howe Island, Tahiti, and Fiji. The body is long and snakelike, roughly tubular and the head is small, with the jaws reaching back to below the eye or further. The dorsal (top) and anal (bottom) fins are of roughly equal length. The colour varies considerably from one individual to another; a deep olive-green is typical but it can be much lighter; golden or even (rarely) yellowish. There are no markings of note, but the underside is pale, often silvery, and the fins greenish. When full grown, they reach about 90 cm. The short-finned eel has a typical regeneration time of 15 to 30 years for females and it reaches a maximum size of about 1.1 m and 3 kg. Males tend to be slower growing and reach a smaller adult size. Anguillid eels are undifferentiated gonochoristic fish. This means that the sex of the animal is determined from an undifferentiated gonad. Differentiation then occurs and an eel becomes male or female, and this is generally correlated to the size (20.0-22.5 cm) of the animal not its age. They are common throughout the lowlands of New Zealand, including both Chatham and Stewart Island/Rakiura, but tend not to ascend as far inland as long-finned eels. In Australia, they are restricted to the area on the seaward side of the Great Dividing Range, from about Mount Gambier in the south-eastern corner of South Australia, through Victoria, Tasmania, the Bass Strait islands and up the eastern seaboard to the Richmond River in northern New South Wales. Unable to scale the Great Divide, and not extending as far west as the outlet of the Murray River, they are excluded from the thousands of miles of waterways that drain inland eastern Australia. A. Australis is the most widely distributed longitudinally of the Anguillid eels where its larvae can be found just south of Fiji to the North-west of Australia in the SEC region (14.5-21°S, 154-179.5°E) (For a comparison with northern hemisphere eels, in particular the European eel, see eel life history.) The larvae drift on the ocean currents and eventually reach coastal waters, where they metamorphose into elvers (tiny, semi-transparent eels). From there, they migrate upstream, traversing numerous obstacles — if necessary, leaving the water and travelling short distances over moist ground. They are well fitted to this task, being able to absorb 50% of the oxygen they need through the skin. Eventually, they take up residence in a lake, swamp, dam or river, typically occupying a home range of about 400 m in length, where they remain until they reach maturity at about 14 years for males and 18 to 24 years for females. They are carnivorous, eating crustaceans, fish, frogs and even small birds. Like the other anguillids, short-finned eels are catadromous: when they reach maturity, they stop feeding and migrate downstream to the sea, then anything up to three or four thousand kilometres to a spawning ground in deep water somewhere in the Coral Sea off New Caledonia. The larvae recruit from the sea as small adults when they lack colour and are transparent-giving them the name 'glass eel'. Tropical species have year round recruitment where temperate species such as the short-finned eel have strong seasonal recruitment. Recent evidence that has utilised analytical microchemical techniques in eel otoliths has suggested that eels are facultatively catadromous rather than obligatory. Discrete populations of ocean and estuarine residents exist, that very rarely enter freshwater. The reproductive biology of these eels has remained elusive during the marine phase. Much is known about the longer freshwater phase from the juvenile to puberal stage, but much less is known about the marine stage Newer research has identified that tropical species such as A. reinhardtii have a shorter larval migration and faster corresponding growth, suggesting a water temperature effect on growth. Like other anguillids, short-finned eels are remarkably hardy: they can tolerate high water temperatures and low oxygen concentrations, endure long periods without food, and bury themselves in mud or sand and enter an energy-saving torpor when the water temperature drops below 10°C. They are one of the few Australian freshwater fish to have coped well with the wholesale introduction of European and American species.