The short-finned eel (''Anguilla australis''), also known as the shortfin eel, is one of the 15
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of
eel
Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
in the family
Anguillidae. It is native to the lakes, dams and coastal rivers of south-eastern
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
,
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
, and much of the
South Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, including
New Caledonia
)
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, image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg
, map_alt = Location of New Caledonia
, map_caption = Location of New Caledonia
, mapsize = 290px
, subdivision_type = Sovereign st ...
,
Norfolk Island
Norfolk Island (, ; Norfuk: ''Norf'k Ailen'') is an external territory of Australia located in the Pacific Ocean between New Zealand and New Caledonia, directly east of Australia's Evans Head and about from Lord Howe Island. Together with ...
,
Lord Howe Island
Lord Howe Island (; formerly Lord Howe's Island) is an irregularly crescent-shaped volcanic remnant in the Tasman Sea between Australia and New Zealand, part of the Australian state of New South Wales. It lies directly east of mainland P ...
,
Tahiti
Tahiti (; Tahitian ; ; previously also known as Otaheite) is the largest island of the Windward group of the Society Islands in French Polynesia. It is located in the central part of the Pacific Ocean and the nearest major landmass is Austr ...
, and
Fiji
Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists ...
.
Description
![Anguilla australis (Short-finned eel)](https://upload.wikimedia.org/wikipedia/commons/d/dd/Anguilla_australis_%28Short-finned_eel%29.gif)
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 () of the animal, not its age.
Distribution and habitat
They are common throughout the lowlands of New Zealand, including both
Chatham
Chatham may refer to:
Places and jurisdictions Canada
* Chatham Islands (British Columbia)
* Chatham Sound, British Columbia
* Chatham, New Brunswick, a former town, now a neighbourhood of Miramichi
* Chatham (electoral district), New Brunswic ...
and
Stewart Island/Rakiura, but tend not to ascend as far inland as
New Zealand longfin eel
The New Zealand longfin eel (''Anguilla dieffenbachii'') is a species of freshwater eel that is endemic to New Zealand. It is the largest freshwater eel in New Zealand and the only endemic species – the other eels found in New Zealand are the n ...
s.
[
In Australia, they are restricted to the area on the seaward side of the ]Great Dividing Range
The Great Dividing Range, also known as the East Australian Cordillera or the Eastern Highlands, is a cordillera system in eastern Australia consisting of an expansive collection of mountain ranges, plateaus and rolling hills, that runs rough ...
, from about Mount Gambier in the south-eastern corner of South Australia
South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
, through Victoria, Tasmania
)
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, image_map = Tasmania in Australia.svg
, map_caption = Location of Tasmania in AustraliaCoordinates:
, subdivision_type = Country
, subdi ...
, the Bass Strait
Bass Strait () is a strait separating the island state of Tasmania from the Australian mainland (more specifically the coast of Victoria, with the exception of the land border across Boundary Islet). The strait provides the most direct waterwa ...
islands, and up the eastern seaboard to the Richmond River in northern New South Wales
)
, nickname =
, image_map = New South Wales in Australia.svg
, map_caption = Location of New South Wales in AustraliaCoordinates:
, subdivision_type = Country
, subdivision_name = Australia
, established_title = Before federation
, es ...
. Unable to scale the Great Dividing Range
The Great Dividing Range, also known as the East Australian Cordillera or the Eastern Highlands, is a cordillera system in eastern Australia consisting of an expansive collection of mountain ranges, plateaus and rolling hills, that runs rough ...
, 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
Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists ...
to the north-west of Australia in the SEC region (14.5–21°S, 154–179.5°E)
During their life cycle however, eels migrate over huge distances to spawn.
Feeding
They are carnivorous, and ferocious predators, eating crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
s, fish, frogs, birds, snakes, and Australian native water rats ( rakali).
Life cycle
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
The Coral Sea () is a marginal sea of the South Pacific off the northeast coast of Australia, and classified as an interim Australian bioregion. The Coral Sea extends down the Australian northeast coast. Most of it is protected by the Fre ...
off New Caledonia
)
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, image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg
, map_alt = Location of New Caledonia
, map_caption = Location of New Caledonia
, mapsize = 290px
, subdivision_type = Sovereign st ...
. 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
Leptocephalus (meaning "slim head") is the flat and transparent larva of the eel, marine eels, and other members of the superorder Elopomorpha. This is one of the most diverse groups of teleosts, containing 801 species in 4 orders, 24 families, ...
("narrow head" larvae), drift on the ocean current
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
s (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
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...
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 in length that they hit puberty
Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. ...
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 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
The Arthur Rylah Institute for Environmental Research (ARI) is the biodiversity research organisation for the government of Victoria, Australia. It provides advice on ecologically sustainable land and water management issues and with regard to th ...
tracked short-finned eels that travelled 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.
Reproductive biology
, 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
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
. They are one of the few Australian freshwater fish to have coped well with the wholesale introduction of European and American species.
As food
Short-finned eels make excellent eating and have long been esteemed as an important food. The consumption of short-finned eels is a longstanding tradition in many Pacific nations, including Japan, Australia and New Zealand.
Prior to European settlement at least two Aboriginal Australian
Aboriginal Australians are the various Indigenous peoples of the Australian mainland and many of its islands, such as Tasmania, Fraser Island, Hinchinbrook Island, the Tiwi Islands, and Groote Eylandt, but excluding the Torres Strait Islands ...
nations, the Gunditjmara and the Djab wurrung
The Djab Wurrung, also spelt Djabwurrung, Tjapwurrung, Tjap Wurrung, or Djapwarrung, people are Aboriginal Australians whose country is the volcanic plains of central Victoria from the Mount William Range of Gariwerd in the west to the Pyrenee ...
from Western Victoria, farmed eels on a large scale, trading smoked eel with distant communities in return for other goods.
For the Māori people
The Māori (, ) are the indigenous Polynesian people of mainland New Zealand (). Māori originated with settlers from East Polynesia, who arrived in New Zealand in several waves of canoe voyages between roughly 1320 and 1350. Over several ce ...
of New Zealand, starved of protein after the extinction of New Zealand megafauna
In terrestrial zoology, the megafauna (from Greek μέγας ''megas'' "large" and New Latin ''fauna'' "animal life") comprises the large or giant animals of an area, habitat, or geological period, extinct and/or extant. The most common threshold ...
, the short-finned eel was a significant food resource. The short-finned eel is known to Māori as , alongside the endemic New Zealand longfin eel
The New Zealand longfin eel (''Anguilla dieffenbachii'') is a species of freshwater eel that is endemic to New Zealand. It is the largest freshwater eel in New Zealand and the only endemic species – the other eels found in New Zealand are the n ...
. They had a highly developed fishery for freshwater eels before the arrival of Europeans
Europeans are the focus of European ethnology, the field of anthropology related to the various ethnic groups that reside in the states of Europe. Groups may be defined by common genetic ancestry, common language, or both. Pan and Pfeil (2004) ...
, and an extensive knowledge of the ecology of eels, harvesting them through a variety of techniques. Present-day recreational anglers catch and eat them regularly, and New Zealand has a well-established commercial eel fishery. Māori are now concerned that the commercial fishery is compromising their ability to harvest short-finned eels for ceremonial purposes.[
]
Decline of wild seed
The main reason that the aquaculture of Anguillid eels has become more popular in recent times is that the wild seed supply cannot support the demand of the consumer. Because of declining catches of both the European and Japanese glass eels, seed from the short-finned eels is at a commercial premium. The increasing pressure on eels has led to reduced recruitment of juveniles of anguilid species which limits the industry and potentially leads to ecological impacts. Supply of seed to aquaculture operations of these eels relies solely on wild stocks of glass eels. These stocks are declining and there is now pressure being placed on the development of intensive eel culture technology. Anguillid eels cannot be reared from eggs in a culture setting, it is the glass eels caught from wild stock that are reared.
Aquaculture of the short-finned eel
History
Extensive systems of aquaculture
Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
to farm the short-finned eel were developed by the Gunditjmara people of western Victoria, Australia
Victoria is a state in southeastern Australia. It is the second-smallest state with a land area of , the second most populated state (after New South Wales) with a population of over 6.5 million, and the most densely populated state in Au ...
, dating to at least 7,000 years ago in the wetlands created by the Tyrendarra lava flow of the Budj Bim
Budj Bim, also known as Mount Eccles, is a dormant volcano near Macarthur in southwestern Victoria, Australia. It lies within the geologically-defined area known as the Newer Volcanics Province, which is the youngest volcanic area in Australi ...
volcano. Today, the areas are protected by three Budj Bim heritage areas.[ See also attached documents: National Heritage List ''Location and Boundary Map'', and ''Government Gazette'', 20 July 2004.] The eel was known as kooyang in the Gunditjmara language
Dhauwurd Wurrung is a term used for a group of languages spoken by various groups of the Gunditjmara people of the Western District of Victoria, Australia. Keerray Woorroong (also spelt Girai Wurrung and variants) is regarded by some as a sepa ...
.
Current industry
Since short-finned eels make excellent eating, there is global demand for them. The major market is Japan with smaller markets in other parts of Asia and in Europe. Aquaculture is the single fastest growing animal food producing sector in the world, having an annual growth rate of 6.9%. The worldwide production of Anguillid eels from aquaculture is estimated to be 242 000 t per annum with a value of $US 1 billion. Chinese eel aquaculture production has increased dramatically over the last 10 years, now accounting for 75% of the total annual world production, and relies on the demand of the Japanese market.
Techniques for artificial breeding of the Japanese eel "A. japonica" have been studied intensively since the 1960s. It is difficult to rear these animals in captivity, the main problem being incomplete techniques for inducing sexual maturation and incomplete rearing techniques of larvae.
However the main way that full growth of oocytes is achieved is by repeated injections of salmon pituitary extracts,[ and also additions of other hormones.
]
Development of the industry
The important factors for developing and growing this industry are having viable and reliable seed sources, as well as understanding the basic biology of the animal. This includes an understanding of the times at which spawning in freshwater occurs as well as maintaining a sustainable catch of any given population.
Eel farming was trialled in New Zealand in the 1970s[ but had limited success. However, the implementation of new technology has made farming a viable option once more, with greater production efficiency. NIWA ( National Institute of Water and Atmospheric Research) has begun developing culture and husbandry techniques in freshwater, and will also be investigating trials in saltwater.
New Zealand is in a unique position where the shortfin eel stock, despite heavy exploitation, show little evidence of decline nationally.][ New Zealand is therefore in a position to enhance global production of shortfin eels and relieve pressure on other stocks around the world in countries such as China, Japan, Korea and Taiwan where aquaculture potential has been limited.][Skehan, B. W., & Silva, S. S. D. (1998). Aspects of the culture-based fishery of the shortfinnedeel, Anguilla australis, in western Victoria, Australia. Journal of Applied Ichthyology, 14(1–2), 23–30.]
Most eel culture facilities utilise freshwater. However a saltwater eel farm operated in Bristol in the United Kingdom for 15 years, using warm seawater growing eels to a marketable size in 18–24 months (it was shut down for economic reasons.)[
Water temperature has a notable effect on the metabolic activity of fish species. Anguillids can withstand a wide variety of temperatures but the optimum temperature in cultured species is between 22 °C and 28 °C- at this temperature growth is maximised] Recent work has determined that the optimal water temperature is for maximal growth of short-finned eels is 26.5 °C, and salinity is optimum at 17.5%. These parameters were shown to give the highest mean specific growth rate of these eels. It has been shown in the European eel that they achieve higher growth when in saline. Much research continues in this area.
Sustainability of aquaculture operations
A major problem in the sustainability of aquaculture of these eels is controlling reproduction and larval development. Culture conditions have a skewed male sex ratio from a small body size. This size difference between sexes is seen in eels housed within a tank. Males are smaller than females and some size difference extends 3g-270g. Many fisheries therefore concentrate on females, except for the Lake Ellesmere (New Zealand) fishery which consists of a two-thirds male catch.[ The goal of many aquaculture operations is to favour female development, the most common way is by the addition of hormones however this is not favoured in all areas of the world.][
]
References
Further reading
* Allen, Midgley & Allen, ''Field Guide to the Freshwater Fishes of Australia'', Western Australian Museum, 2002. .
* National Institute of Water & Atmospheric Research,
Atlas of New Zealand Freshwater Fishes
'.
* National Institute of Water and Atmospheric Research,
"Encouraging results from freshwater eel culture"
'
* Beumer, J P Feeding and movement of Anguilla australis and A.reinhardtii in Macleods Morass, Victoria, Australia. Journal of Fish Biology, 1979,14:573–592
External links
*
*
Native Fish Australia: short-finned eel
{{DEFAULTSORT:Short-Finned Eel
Anguillidae
Freshwater fish of Australia
Freshwater fish of New Zealand
Taxa named by John Richardson (naturalist)
Fish described in 1841
New Zealand seafood
Māori cuisine