We rarely see koaven or freshwater eels that inhabit in our fresh water lakes (kilhi). We spot them mostly in aaro, the passages (flood tunnels) that have been created in our island to pass water out of the island during periods of heavy rain fall or when the lakes (kilhi) overflow. People say that these eels don’t bite. But since early days there is a “myth” that these creatures have a “rasp” tongue that licks if it comes in contact with a human being. I searched the internet to clarify this and found no fact to support this this saying. But I was able to know that they have rasping teeth and are jawless.
I was also able to know that the French Scientist Charles Richet won his noble price in 1913, when he discovered anaphylaxis, life threatening allergic reaction, while he was researching the toxin derived from the blood serum of fresh water eels. We rarely see this species. The eels are seen during heavy flooding in taro fields, aaro and shallow waters.
These species are of the Anguillidae family of fishes that contains the freshwater eels. The 19 species and six subspecies in this family are all in genus Anguilla. They are catadromous, meaning they spend their lives in freshwater rivers, lakes, or estuaries, and return to the ocean to spawn. The young eel larvae, called leptocephali, live only in the ocean and consume small particles called marine snow.
They grow larger in size, and in their next growth stage, they are called glass eels. At this stage, they enter estuaries, and when they become pigmented, they are known as elvers. Elvers travel upstream in freshwater rivers, where they grow to adulthood. Some details of eel reproduction are as yet unknown, and the discovery of the spawning area of the American and European eels in the Sargasso Sea is one of the more famous anecdotes in the history of ichthyology. The spawning areas of some other anguillid eels, such as the Japanese eel, and the giant mottled eel, were also discovered recently in the western North Pacific Ocean.
Freshwater eels are elongated with tubelike, snake-shaped bodies. They have large, pointed heads and their dorsal fins are usually continuous with their caudal and anal fins, to form a fringe lining the posterior end of their bodies. They have small pectoral fins to help them navigate along river bottoms. Their scales are thin and soft. Freshwater eels go through physical changes in their bodies when going to and from the ocean for different stages of life.
Anguillid eels are important food fish. Eel aquaculture is a fast-growing industry. Important food eel species include longfin eel, Australian long-finned eel, short-finned eel, and Japanese eel. Most eel production historically has been in Japan, Korea, andTaiwan, but in recent years, the greatest production has been in China.
Two main types
There are two main types of eel – the shortfin and the longfin. There are fewer eels today because of the loss of wetlands and historical commercial fishing practice.
They live mainly in rivers and inland lakes but can be found in almost all types of waters, usually well inland from the coast. They are legendary climbers and have made their way well inland in most river systems, even those with natural barriers. Elvers (young eels) swimming up river will climb waterfalls and even dams by leaving the water and wriggling over damp areas. It is not unheard of for an eel to climb a waterfall of up to 20 metres.
When eels begin life, they are a tiny one millimetre in length. During their life, they can grow up to two metres long. Compared with many other fish, eels are slow growing – a longfin may grow only between 15-25mm a year. They can also live for many years. Large longfins have been estimated to be at least 60 years old. The biggest eels are usually old females that have been slow to reach sexual maturity and, for reasons that are not yet understood, have not migrated to sea to breed. The biggest longfin eels reported have weighed as much as 40 kg. Pictures of fishers and huge eels used to appear regularly in local newspapers. But today, you’ll seldom find an eel heavier than 10 kg. Commercial fishing has meant that a big proportion of our very large eels have now disappeared.
In shape, eels are elongate, slender-bodied fishes, almost tubular. When they are small, they have relatively smooth heads but as they grow the head becomes bulbous, with a prominent muscular dome behind the eyes. They change shape again when they get ready to migrate to their breeding grounds. The head becomes much more slender and tapered, almost bullet-like and the eyes enlarge to up to twice their normal size.
Like all fish, eels have scales and fins. The longfin eel is so named because its top (dorsal) fin is longer than its bottom fin. While they have the appearance of being scaleless, tiny scales are embedded deeply within their thick, leathery skin. The eel’s skin is very sensitive to touch. This helps it to “see” in its watery environment. In colour, longfins are usually dark brown to grey black. Very occasionally, longfin eels found in the wild are partially or even wholly bright yellow in colour.
Eels eat “live” food. Small longfin eels living amongst the river gravels will feed on insect larvae, worms and water snails. When they get bigger, they begin to feed on fish. They will also eat fresh-water crayfish and even small birds like ducklings. During the day, eels are secretive, hiding under logs and boulders or under riverbanks. Occasionally, they may be seen out hunting for food but most of their hunting takes place at night. Eels hunt by smell rather than sight. Longfin eels have a well-developed sense of smell. They have tube nostrils that protrude from the front of their head, above their upper lip. They also have a very large mouth with rows of small, sharp, white teeth. The top teeth form an arrow shape on the roof of the eel’s mouth.
Longfin eels breed only once, at the end of their life. When they are ready to breed, they leave New Zealand and swim 5000 kilometres up into the tropical Pacific to spawn, probably in deep ocean trenches somewhere near Tonga. When they reach their destination, the females lay millions of eggs that are fertilised by the male. The larvae are called leptocephalus and look nothing like an eel – they are transparent, flat, and leaf-shaped. Before entering fresh water, the leptocephalus change into a more familiar eel shape, although they remain transparent for up to a week after leaving the sea. These tiny “glass” eels enter fresh water between July and November each year, often in very large numbers. Eels take many years to grow and it could be decades before an individual is ready to undertake the long migration back to the tropics to breed. The average age at which a longfin eel migrates is 23 years for a male and 34 for a female. The adults never return as they die after spawning.
While longfin eels are still relatively common, fishing has had a significant impact on the species. Europeans showed little interest in eels as a fishery until the 1960s when commercial catches rose steadily. In 1975, eels were the most valuable fish export after rock lobsters. Five years later, they were the fifth most valuable finfish export. This big increase in fishing effort led to significant stock reductions in some areas, with a marked decline in the average size of the eels caught. Habitat loss also affects eels. Changes caused by hydro development, drainage and irrigation schemes and river diversions affect eels by reducing their habitat and the water available for aquatic life. Culverts and dams can also impact on eels by preventing their migration. Eel habitat is also impacted by pollution. Sewage and effluent from meat works and pulp and paper plants discharged into rivers can remove large quantities of oxygen from the water. The result of this oxygen depletion is that the fish will either die or move away.
Longfin Eel, Anguilla reinhardtii Steindachner, 1867
The Longfin Eel is a large, primarily carnivorous fish that has a broad head, and a large mouth with fleshy lips.
Standard Common Name
Australian Longfinned Eel, Conger Eel, Freshwater Eel, Marbled Eel, River Eel, Spotted Eel
The Longfinned Eel has well developed pectoral fins, a broad head, and a large mouth with fleshy lips. It can be distinguished from the similar-looking Shortfinned Eel, Anguilla australis, by the length of the dorsal fin. The dorsal fin origin of the Longfinned Eel is well forward of the anal fin origin, whereas the dorsal fin origin of the Shortfinned Eel is above the anal fin origin. The two species also differ in dentition and colouration. The Longfinned Eel usually has olive or brown blotches above and on the sides, fading to pale on the belly. The median fins are brown and the pectoral fins are often yellowish. The Shortfinned Eel is usually a uniform colour and does not have a blotched pattern. The long-finned eel is so named because of its long dorsal fin that extends all the way from the mid-back to the tip of the tail (Allen 1989). It can be distinguished from the short-finned eel by the distance foward of the anal fin it extends. They are olive-green in colour, and are often spotted or mottled. Males of this species are smaller than the females, who reach an average length of 1 metre; some landlocked eels who cannot participate in the migration have been found to reach up to 3 metres in length . The eels inhabit freshwater lakes, ponds, rivers and estuaries.
Landlocked Longfin Eels (those that cannot return to sea, due to physical barriers) can grow to 3 m in length and weigh 22 kg. The species is usually seen at much smaller sizes than this, often about 1m. Males are smaller than females.
The species occurs in Australia, New Caledonia and New Zealand. In Australia, it is known from the entire coastal margin of eastern Australia from Cape York to Melbourne and also from northern and eastern Tasmania and Lord Howe Island.
The Longfinned Eel lives in rivers, lakes and swamps, but appears to prefer flowing water.
Danger to humans and first aid
Eel blood is toxic (if injected) to humans and other mammals, but both cooking and the digestive process destroy the toxic protein. The toxin derived from eel blood serum was used by Charles Richet in his Nobel winning research which discovered anaphylaxis (by injecting it into dogs and observing the effect).” Injection for Anaphylaxis, a serious allergic reaction that is rapid in onset and may cause death.
What’s the difference?
Longfin eel (Anguilla dieffenbachii) and the shortfin eel (Anguilla australis).
On a longfin eel, the dorsal (top) fin extends a lot further forward than the anal (bottom) fin. The dorsal fin of a shortfin eel only extends a little further forward than the anal fin.
Diagram of eels with longfin eel at top and shortfin eel below
A species in decline
The longfin eel is ranked as ‘At Risk – Declining’ in the New Zealand Threat Classification System listings (2009). It is only found in New Zealand. The shortfin eel is ‘Not Threatened’ and is found in New Zealand, Australia and some Pacific Islands.
Longfin eels, as well as being rare, are less able to cope with changes to their environment than their shorter-finned relative. They are heavily affected by human activities, such as pollution, the building of dams, loss of vegetation near their habitat, and overfishing.
Anyone who has handled an eel out of water knows that they will not usually stay still long enough to allow you to measure their fins! So some ‘rules of thumb’ can be used to identify them.
An eel is probably a longfin if it’s:
- very dark in colour
- more than a metre long
- living in a high-country river or lake, or a clear, cold, spring-fed stream at any altitude.
If in doubt, there is a very reliable way to tell the difference (see images on right):
- a longfin’s skin forms big, loose, obvious wrinkles when bent
- a shortfin’s skin wrinkles are much smaller.
The short-finned eel is distinguishable from the long-finned eel by close examination of the origin of the dorsal fin to see whether or not it approximately originates close to the anal fin. The eels are olive-brown in colour with silvery-white bellies and can reach up to 1.1m in length. In direct contrast to the long-finned eel, short-finned eels prefer stillwater habitats such as ponds or lakes
Summary of differences
|Shortfin eel||Longfin eel|
|Found in New Zealand, eastern Australia and some Pacific Islands||Found only in New Zealand|
|Range of colours, often light brown, olive||Usually dark brown/black|
|Grows up to 1 m long and weighs up to 3.5 kg||Often more than 1 m long and can grow up to 2 m long, and can weigh up to 20 kg, sometimes more (although this is now very rare)|
|Small wrinkles on the skin when bent||Big, loose wrinkles on the skin when bent|
|Lives mostly in lowland areas||Lives at a wide range of altitudes, including very high elevations|
|Relatively pollution tolerant||Relatively intolerant of pollution|
|Lives for an average of 18–23 years and up to around 60 years||Lives for an average of 35–52 years and up to 100 years, sometimes more|
Did you know?
New Zealand freshwater eels can live up to 100 years and breed only once at the end of their lives. In order to breed, they undergo mass spawning migrations, leaving the familiarity of lakes and rivers to swim all the way up to the subtropical Pacific Ocean, where they spawn en masse in very deep water.