Shortfin eel
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 of the adult short-finned eel 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 (30 cm (12 in)) 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 New Zealand longfin 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 Dividing Range, and not extending as far west as the outlet of the Murray River, they are excluded from the thousands of kilometres 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 Southern Equatorial Current region (14.5–21°S, 154–179.5°E).
During their life cycle however, eels migrate over huge distances to spawn.
They are carnivorous, and ferocious predators, eating crustaceans, fish, frogs, birds, snakes, and Australian native water rats (rakali).
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, whereas 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 leptocephalus ("narrow head" larvae), drift on the ocean currents (off Australia, the South Equatorial Current and eventually reach coastal waters, where they metamorphose into "glass eels" (tiny, semi-transparent eels). At this stage they are unable to eat, and can only eat after reaching an estuary or river mouth, where they metamorphose again into elvers, which are darker in colour. From there, they migrate upstream, traversing numerous obstacles — if necessary, leaving the water and travelling short distances over moist ground. They are well-suited to this task, being able to absorb some of the oxygen they need through the skin.
It is only when they reach about 30 cm (12 in) in length that they hit puberty and it is only then that their sex, which depends on population density, is determined. In an area with many eels, they tend to turn into males, whereas further upstream, where there are fewer eels, they are more likely to become female. Eventually, they take up residence in a lake, swamp, dam or river, typically occupying a home range of about 400 m (1,300 ft) in length, where they remain until they reach maturity at about 14 years for males and 18 to 24 years for females.
At some point, the eels start changing their shape again, to prepare for a trip back into the ocean for spawning. Their digestive system shrinks and their gonads become larger, to make room for the eggs and sperm that they will be making, and they stop eating. Their eyes get bigger, and their heads pointier, possibly an adaptation for better ocean swimming. In the case of the south-east Australian eels, they head north up the east coast, heading for the Coral Sea. A 2021 study by the Arthur Rylah Institute tracked short-finned eels that travelled 2,620 km (1,630 mi) from western Victoria up the east coast to the Coral Sea. The study did not establish where the eels spawned, but the researchers thought that it was somewhere near New Caledonia.
As of 2000, 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 pubertal stage, but much less is known about the marine stage Research done in 2008 identified that tropical species such as A. reinhardtii (Australian long-finned eels) 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 introduction of European and American species.
