Although snake-like in appearance, freshwater eels are amazing ray-finned fish that complete part of their life cycle in our waterways, dams and wetlands. The Short-finned Eel (Anguilla australis) and Long-finned Eel (Anguilla reinhardtii) are found in south-east Queensland (SEQ) and are listed internationally as threatened.
In SEQ, the most commonly encountered is the Long-finned Eel, which can reach an impressive 2.5 metres in length and 20 kgs in weight, but most reports are of individuals about one metre long. Both eel species prefer still waters such as lakes, dams and swamps where they feed on all manner of aquatic animals including fish, insects, yabbies, shrimps, molluscs, frogs, small waterbirds and even the odd Rakali.
Eels are remarkably hardy and can tolerate high water temperatures, 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 non-native fish species.
Photos by Greg Tasney.
The lifecycle of eels is like something out of a science fiction movie! Eels reach puberty when they are about 30 cm in length and it is only then that their sex, which depends on population density, is determined. In an area with many eels, they tend to become males, whereas further upstream, where there are fewer eels, they are more likely to become female. Once in puberty, they remain in the same area until they reach maturity at about 14-25 years for males and 18-35 years for females. Once fully mature, the eels start changing their shape. Their digestive system shrinks and they stop eating. Their eyes get bigger and their heads pointier, possibly an adaptation for long distance swimming.
The mature adults migrate from their freshwater home down to the sea in order to spawn. Incredibly, they navigate for 2500 km (over 4000 km for the ones coming from Tasmania) to the western side of New Caledonia and Vanuatu. Their journey takes 4-6 months, swimming against the East Australian Current without eating any food! Recent studies have found that they swim at a depth of about 80-100 metres at night and dive to depths of 900-1000 metres during the day and on full moon nights. One theory speculates they do this to avoid predation by mainly visual hunters like sharks and other predatory fish.
When they reach their destination, spawning takes place deep in the ocean, on the western side of the New Caledonian submarine ridge at a depth of about 350m. Mature females have been found to contain more than 3 million eggs.
The eel larvae, known as leptocephali because of their leaf like flat shape, are carried south by the East Australian Current from their spawning grounds until they reach Australia’s continental shelf. At around this time they metamorphose into the normal tubular eel shape although devoid of any pigment and so are known as glass eels.
When the glass eels begin to migrate into freshwater they may be anywhere from one to three years old. While still in the estuarine waters the glass eels quickly develop pigment (called elvers) and adjust to freshwater. These migrations are known as ‘eel fares’ from which the term ‘elver’ is derived. The upstream migration continues well into the upper reaches of river systems. Many elvers, glass eels and adults can overcome large obstructions such as dams and waterfalls by travelling overland in damp conditions with a motion much like snakes.
So, next time you see one of these snake-like fish in your dam or creek, spare a thought for the amazing journey it took for them to reach this spot, and even more, the amazing journey it will take for them to breed.
Article by Stefan Hattingh
Land for Wildlife Officer
City of Moreton Bay
Reference and Further Reading
Clancy N (2010) Close eel-encounters of a slippery kind…the amazing life cycle of eels. LFWSEQ Newsletter, January.
Inoue JG, Miya M, Miller MJ, Sado T et. al. (2010) Deep-ocean origin of the freshwater eels. Biology Letters 6: 363-366.
Koster WM, Aarestrup K, Birnie-Gauvin K et. al. (2021) First tracking of the oceanic spawning migrations of Australasian short-finned eels (Anguilla australis). Scientific Reports 11.