Eel life history

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Distribution and size of leptocephali larvae of the American eel, Anguilla rostrata Rostratamuk.jpg
Distribution and size of leptocephali larvae of the American eel, Anguilla rostrata

Eels are any of several long, thin, bony fishes of the order Anguilliformes. They have a catadromous life cycle, that is: at different stages of development migrating between inland waterways and the deep ocean. Because fishermen never caught anything they recognized as young eels, the life cycle of the eel was long a mystery. Of particular interest has been the search for the spawning grounds for the various species of eels, and identifying the population impacts of different stages of the life cycle.

Contents

Past studies of eels

The European eel (Anguilla anguilla) is the one most familiar to Western scientists, beginning with Aristotle, who wrote the earliest known inquiry into the natural history of eels. He speculated that they were born of "earth worms", which he believed were formed of mud, growing from the "guts of wet soil" rather than through sexual reproduction. Many centuries passed before scientists were able to demonstrate that such spontaneous generation does not occur.

In 1777, the Italian Carlo Mondini located an eel's ovaries and demonstrated that eels are a kind of fish. [1] In 1876, as a young student in Austria, Sigmund Freud dissected hundreds of eels in search of the male sex organs. He had to concede failure in his first major published research paper, and turned to other issues in frustration. [2] [3] [4] [5]

Larval eels — transparent, leaflike two-inch (five-cm) creatures of the open ocean — were not generally recognized as eels until 1893; instead, they were thought to be a separate species, Leptocephalus brevirostris (from the Greek leptocephalus meaning "thin- or flat-head"). In 1886, however, the French zoologist Yves Delage discovered the truth when he kept leptocephali alive in a laboratory tank in Roscoff until they matured into eels, and in 1896 Italian zoologist Giovanni Battista Grassi confirmed the finding when he observed the transformation of a Leptocephalus into a round glass eel in the Mediterranean Sea. (He also observed that salt water was necessary to support the maturation process.) Although the connection between larval eels and adult eels is now well understood, the name leptocephalus is still used for larval eel.

Search for the spawning grounds

Leptocephalus larva of an ocean eel LeptocephalusConger.jpg
Leptocephalus larva of an ocean eel
Glass eels at the transition between ocean and fresh water; the skin is still transparent and the red gills and the heart are visible; length about 8 cm Glasseelskils.jpg
Glass eels at the transition between ocean and fresh water; the skin is still transparent and the red gills and the heart are visible; length about 8 cm
Juvenile eels, length about 25 cm Rostrata.jpg
Juvenile eels, length about 25 cm

European eel

The Danish professor Johannes Schmidt, beginning in 1904, led a series of expeditions into the Mediterranean Sea and the North Atlantic (the Dana expeditions) to investigate eels. The expeditions were largely financed by the Carlsberg Foundation. He noted that all the leptocephali he found were very similar, and hypothesized that they all must have descended from a common ancestor. He also observed that the farther out to sea in the Atlantic Ocean he went, the smaller the leptocephali were. In a 1922 expedition, he sailed as far as the Sargasso Sea, south of Bermuda, where he caught the smallest eel-larvae that had ever been seen. Although Schmidt did not directly observe eel spawning, or even find ready-to-spawn adult eels, he deduced the following about the life history of the eel, based on the size distribution of the leptocephali he collected: The larvae of European eels travel with the Gulf Stream across the Atlantic Ocean from the Sargasso Sea, and grow to 75–90 mm within one to three years, before they reach the coasts of Europe. Marine eels of the order Anguilliformes also have a leptocephalus stage, and likely pass through a stage similar to the anguillid glass eels, but they are rarely seen in the ocean.

Eels in this so-called "recruitment" developmental stage are known as glass eels because of the transparency of their bodies. The term typically refers to a transparent eel of the family Anguillidae. It is applied to an intermediary stage in the eel's complex life history between the leptocephalus stage and the juvenile (elver) stage. Glass eels are defined as "all developmental stages from completion of leptocephalus metamorphosis until full pigmentation". [6] Once the glass eels arrive at coastal areas, they migrate up rivers and streams, overcoming various natural and man-made challenges — sometimes by piling up their bodies by the tens of thousands to climb over obstacles[ citation needed ] — and they reach even the smallest of creeks. At this stage in their growth they are small enough to benefit from surface tension in order to climb vertical walls. [7]

In fresh water they develop pigmentation, turn into elvers (young eels), and feed on creatures such as small crustaceans, worms, and insects. For 10 to 14 years they mature, growing to a length of 60 to 80 cm. The eels can propel themselves over wet grass and dig through wet sand to reach upstream headwaters and ponds, thus colonizing the continent. During this stage they are called yellow eels because of their golden pigmentation.

In July, some mature individuals migrate back towards the sea, sometimes crossing wet grasslands at night to reach rivers that lead to the sea. Eel migrations out of their freshwater growth habitats from various parts of Europe, or through the Baltic Sea in the Danish straits, have been the basis of traditional fisheries with characteristic trapnets.

Details of the adults' migration across 6,000 km (3,700 mi) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remain poorly understood. By the time they leave Europe, their gut dissolves, making feeding impossible, so they have to rely on stored energy alone. [8] The external features undergo other dramatic changes, as well: the eyes start to enlarge, the eye pigments change for optimal vision in dim blue clear ocean light, and the sides of their bodies turn silvery, to create a countershading pattern which makes them difficult to see by predators during their long open-ocean migration. These migrating eels are typically called "silver eels" or "big eyes".

German fisheries biologist Friedrich Wilhelm Tesch conducted many expeditions with high-tech instrumentation to follow eel migration, first down the Baltic, then along the coasts of Norway and England, but finally the transmitter signals were lost at the continental shelf when the batteries ran out [ when? ] According to Schmidt, a travel speed in the ocean of 15 km per day can be assumed, so a silver eel would need around 140 to 150 days to reach the Sargasso Sea from Scotland and about 165 to 175 days when leaving from the English Channel.

Tesch — like Schmidt — kept trying to persuade sponsors to provide more funding for expeditions. His proposal was to release 50 silver eels from Danish waters, with transmitters that would detach from the eels each second day, float up toward the surface, and broadcast their position, depth, and temperature to satellite receivers. He also suggested that countries on the western side of the Atlantic could perform a similar release experiment at the same time. In December 2018 researchers in the Azores, (about 1,400 km (870 mi) west of the Iberian coast—the furthest point on the migration route identified in previous experiments) fitted 26 large female European eels with satellite tags and released them into the Atlantic Ocean. Tracking demonstrated that the fishes' journey to the Sargasso took a further year, or more. [9]

American eel

Another Atlantic eel species is known: the American eel, Anguilla rostrata. First it was believed European and American eels were the same species due to their similar appearance and behavior, but they differ in chromosome count and various molecular genetic markers, and in the number of vertebrae, A. anguilla counting 110 to 119 and A. rostrata 103 to 110.

The spawning grounds for the two species are in an overlapping area of the southern Sargasso Sea, with A. rostrata apparently being more westward than A. anguilla. This was confirmed in 2023. [10] After spawning in the Sargasso Sea and moving to the west, the leptocephali of the American eel exit the Gulf Stream earlier than the European eel and begin migrating into the estuaries along the east coast of North America between February and late April at an age around one year and a length around 60 mm.

Japanese eel

The spawning area of the Japanese eel, Anguilla japonica, has also been found. Their breeding site is to the west of the Suruga seamount (14–17°N, 142–143°E), near the Mariana Islands. [11] and their leptocephali are then transported to the west to East Asia by the North Equatorial Current.

In June and August 2008, Japanese scientists discovered and caught matured adult eels of A. japonica and A. marmorata in the West Mariana Ridge. [12]

Southern African eels

Southern Africa's four species of freshwater eels ( A. mossambica , A. bicolor bicolor , A. bengalensis labiata , and A. marmorata ) have an interesting migratory pattern: It takes them on a long journey from their spawning grounds in the Indian Ocean north of Madagascar to high up in some of the Southern African river systems and then back again to the ocean off Madagascar. [13]

New Zealand longfin eels

New Zealand longfin eels breed only once at the end of their lives, making a journey of thousands of kilometres from New Zealand to their spawning grounds near Tonga. [14] [15] Their eggs (of which each female eel produces between 1 and 20 million) are fertilized in an unknown manner, but probably in deep tropical water. [16] The mature eels then die, their eggs floating to the surface to hatch into very flat leaf-like larvae (called leptocephalus) that then drift along large oceanic currents back to New Zealand. [14] [17] This drifting is thought to take up to 15 months. [16] There have been no recorded captures of either the eggs or larvae of longfin eels. [14]

Decline of the glass eels

Glass eel on the online in situ microscope at the LEO project Rostrataluk.jpg
Glass eel on the online in situ microscope at the LEO project
Glass eel Glasseelkils.gif
Glass eel

For unknown reasons, beginning in the mid-1980s, glass eel arrival in the spring dropped drastically—in Germany to 10% and in France to 14% of their previous levels—from even conservative estimates. Data from Maine and other North American coasts showed similar declines, although not as drastic.[ citation needed ]

In 1997, European demand for eels could not be met for the first time ever, and dealers from Asia bought all they could. The traditional European stocking programs could not compete any longer: each week, the price for a kilogram of glass eel went up another US$30. Even before the 1997 generation hit the coasts of Europe, dealers from China alone placed advance orders for more than 250,000 kg, some bidding more than $1,100 per kg. Asian elvers have sold in Hong Kong for as much as $5,000 to $6,000 a kilogram at times when $1,000 would buy the same amount of American glass eels at their catching sites. [18] Such a kilogram, consisting of 5000 glass eels, may bring at least $60,000 and as much as $150,000 after they leave an Asian fish farm. In New Jersey, over 2000 licenses for glass eel catch were issued and reports of 38 kg per night and fisherman have been made, although the average catch is closer to 1 kg.

Glass eels have been harvested for food from the River Severn, England, for centuries, but for about 200 years, from the sixteenth to eighteenth century, the practice was outlawed by act of Parliament. [19] The restriction was removed in 1873 [20] and in 1908 a collection point and holding station for the catch was established at Epney, Gloucestershire. Initially the crop was sold for human consumption but, as infrastructure for live transport improved, the glass eels were exported throughout Europe for stocking natural waterways and to the Far East for eel aquaculture. [21]

The demand for adult eels has continued to grow, as of 2003. Germany imported more than $50 million worth of eels in 2002. In Europe, 25 million kg are consumed each year, but in Japan alone, more than 100 million kg were consumed in 1996. As the European eels become less available, worldwide interest in American eels has increased dramatically.

New high-tech eel aquaculture plants are appearing in Asia, with possible effects on the native Japanese eel, A. japonica. Traditional eel aquaculture operations rely on wild-caught elvers, but experimental hormone treatments in Japan have led to artificially spawned eels. Eggs from these treated eels have a diameter of about 1 mm, and each female can produce up to 10 million eggs. However, these treated eels may not solve the eel crisis. Scientists are struggling to get eels to sexual maturity without environmental cues. [22] Additionally, leptocephali (larva) require a diet of marine snow which is difficult to recreate in aquaculture. [22]

Threats to eels

Strong concerns exist that the European eel population might be devastated by a new threat: Anguillicola crassus , a foreign parasitic nematode. This parasite from East Asia (the original host is A. japonica) appeared in European eel populations in the early 1980s. Since 1995, it also appeared in the United States (Texas and South Carolina), most likely due to uncontrolled aquaculture eel shipments. In Europe, eel populations are already from 30% to 100% infected with the nematode. Recently, this parasite was shown to inhibit the function of the swimbladder as a hydrostatic organ. [23] [24] As open ocean voyagers, eels need the carrying capacity of the swimbladder (which makes up 3–6% of the eel's body weight) to cross the ocean on stored energy alone.

Because the eels are catadromous (living in fresh water but spawning in the sea), dams and other river obstructions can block their ability to reach inland feeding grounds. Since the 1970s, an increasing number of eel ladders have been constructed in North America and Europe to help the fish bypass obstructions.

As of the early 2000s, in New Jersey, the Long-term Ecosystem Observatory was monitoring glass eel migration with a planned online in situ microscope. [25]

See also

Related Research Articles

<span class="mw-page-title-main">Fish migration</span> Movement of fishes from one part of a water body to another on a regular basis

Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres. Such migrations are usually done for better feeding or to reproduce, but in other cases the reasons are unclear.

<span class="mw-page-title-main">Leptocephalus</span> Juvenile eel

A leptocephalus 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, and 156 genera. This group is thought to have arisen in the Cretaceous period over 140 million years ago.

<span class="mw-page-title-main">European eel</span> Species of fish

The European eel is a species of eel. Their life history was a mystery for thousands of years, and mating in the wild has not yet been observed. The five stages of their development were originally thought to be different species. They are critically endangered due to hydroelectric dams, overfishing by fisheries on coasts for human consumption, and parasites.

<span class="mw-page-title-main">American eel</span> Species of fish

The American eel is a facultative catadromous fish found on the eastern coast of North America. Freshwater eels are fish belonging to the elopomorph superorder, a group of phylogenetically ancient teleosts. The American eel has a slender, supple, snake-like body that is covered with a mucus layer, which makes the eel appear to be naked and slimy despite the presence of minute scales. A long dorsal fin runs from the middle of the back and is continuous with a similar ventral fin. Pelvic fins are absent, and relatively small pectoral fins can be found near the midline, followed by the head and gill covers. Variations exist in coloration, from olive green, brown shading to greenish-yellow and light gray or white on the belly. Eels from clear water are often lighter than those from dark, tannic acid streams.

<span class="mw-page-title-main">Short-finned eel</span> Species of fish

The short-finned eel, 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.

<span class="mw-page-title-main">Aquatic animal</span> Animal that lives in water for most or all of its lifetime

An aquatic animal is any animal, whether vertebrate or invertebrate, that lives in bodies of water for all or most of its lifetime. Aquatic animals generally conduct gas exchange in water by extracting dissolved oxygen via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are evolved from terrestrial ancestors that re-adapted to aquatic environments, in which case they actually use lungs to breathe air and are essentially holding their breath when living in water. Some species of gastropod mollusc, such as the eastern emerald sea slug, are even capable of kleptoplastic photosynthesis via endosymbiosis with ingested yellow-green algae.

<span class="mw-page-title-main">Atlantic salmon</span> Species of fish

The Atlantic salmon is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are found in the northern Atlantic Ocean and in rivers that flow into it. Most populations are anadromous, hatching in streams and rivers but moving out to sea as they grow where they mature, after which the adults seasonally move upstream again to spawn.

<span class="mw-page-title-main">Anguillidae</span> Family of fishes

The Anguillidae are a family of ray-finned fish that contains the freshwater eels. Except from the genus Neoanguilla, with the only known species Neoanguilla nepalensis from Nepal, all the extant species and six subspecies in this family are in the genus Anguilla, and are elongated fish of snake-like bodies, with long dorsal, caudal and anal fins forming a continuous fringe. They are catadromous, spending their adult lives in freshwater, but migrating to the ocean to spawn.

<span class="mw-page-title-main">New Zealand longfin eel</span> Species of fish

The New Zealand longfin eel, also known as ōrea, 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 native shortfin eel, also found in Australia, and the naturally introduced Australian longfin eel. Longfin eels are long-lived, migrating to the Pacific Ocean near Tonga to breed at the end of their lives. They are good climbers as juveniles and so are found in streams and lakes a long way inland. An important traditional food source for Māori, who name them ōrea, longfin eel numbers are declining and they are classified as endangered, but over one hundred tonnes are still commercially fished each year.

<i>Anguillicoloides crassus</i> Species of roundworm

Anguillicoloides crassus is a parasitic nematode worm that lives in the swimbladders of eels and appears to spread easily among eel populations after introduction to a body of water. It is considered to be one of the threats to the sustainability of populations of European eel. It was introduced to the European continent in the 1980s, where it was reported independently from Germany and Italy in 1982, having probably been introduced from Taiwan. It is thought to have reached England in 1987 from continental Europe. It is a natural parasite of the Japanese eel in its native range.

<span class="mw-page-title-main">Atlantic tarpon</span> Species of fish

The Atlantic tarpon is a ray-finned fish that inhabits coastal waters, estuaries, lagoons, and rivers. It is also known as the silver king. It is found in the Atlantic Ocean, typically in tropical and subtropical regions, though it has been reported as far north as Nova Scotia and the Atlantic coast of southern France, and as far south as Argentina. As with all elopiformes, it spawns at sea. Its diet includes small fish and crustaceans.

<span class="mw-page-title-main">Japanese eel</span> Species of fish

The Japanese eel is a species of anguillid eel found in Japan, Korea, Taiwan, China, and Vietnam, as well as the northern Philippines. Like all the eels of the genus Anguilla and the family Anguillidae, it is catadromous, meaning it spawns in the sea but lives parts of its life in freshwater. Raised in aquaculture ponds in most countries, the Japanese eel makes up 95% of the commercially sold eel in Japan, the other 5% is shipped over by air to the country from Europe. This food in Japan is called unagi; they are an essential part of the food culture, with many restaurants serving grilled eel called kabayaki. However, presumably due to a combination of overfishing and habitat loss or changing water conditions in the ocean interfering with spawning and the transport of their leptocephali this species is endangered.

<span class="mw-page-title-main">Johannes Schmidt (biologist)</span> Danish biologist

Ernst Johannes Schmidt was a Danish biologist credited with discovering in 1920 that European eels migrate to the Sargasso Sea to spawn. Before this people in North America and Europe had wondered where the small glass eels, or elvers, came from.

<span class="mw-page-title-main">Giant mottled eel</span> Species of fish

The giant mottled eel, also known as the marbled eel, is a species of tropical anguillid eel that is found in the Indo-Pacific and adjacent freshwater habitats.

<span class="mw-page-title-main">Eel</span> Order of fishes

Eels are ray-finned fish belonging to the order Anguilliformes, which consists of eight suborders, 20 families, 164 genera, and about 1000 species. Eels undergo considerable development from the early larval stage to the eventual adult stage and are usually predators.

<span class="mw-page-title-main">American conger</span> Species of fish

The American conger is a species of eel in the family Congridae. Other common names for this fish include conger, dog eel, poison eel and sea eel. It is a marine fish with a widespread distribution in the Western Atlantic from Cape Cod in Massachusetts to northeastern Florida in United States and the northern Gulf of Mexico, and is also reported from near the mid-Atlantic island of St. Helena and off the coast of Nova Scotia in Canada. Dark grayish color, it can grow to about 6.5 feet (2.0 m) long and weigh more than 40 kg (88 lb).

The following outline is provided as an overview of and topical guide to fisheries:

<span class="mw-page-title-main">Juvenile fish</span> Young fish

Fish go through various life stages between fertilization and adulthood. The life of fish start as spawned eggs which hatch into immotile larvae. These larval hatchlings are not yet capable of feeding themselves and carry a yolk sac which provides stored nutrition. Before the yolk sac completely disappears, the young fish must mature enough to be able to forage independently. When they have developed to the point where they are capable of feeding by themselves, the fish are called fry. When, in addition, they have developed scales and working fins, the transition to a juvenile fish is complete and it is called a fingerling, so called as they are typically about the size of human fingers. The juvenile stage lasts until the fish is fully grown, sexually mature and interacting with other adult fish.

<span class="mw-page-title-main">Short-tooth sawpalate</span> Species of fish

The short-tooth sawpalate is an eel in the family Serrivomeridae. It was described by Johannes Schmidt in 1916 in its larval form, originally under the genus Leptocephalus, and later as a subspecies of Serrivomer sector by Roule & Bertin in 1929. It is a marine, deep water-dwelling eel which is known from the eastern central and western central Atlantic Ocean, including the Strait of Gibraltar, Cape Verde, the United States, the Bahamas and Bermuda, as well as the Strait of Gibraltar, Cape Verde, Canada and the United States. It dwells at a depth range of 150 to 6,000 metres. Males can reach a maximum total length of 65 centimetres (26 in).

<span class="mw-page-title-main">Freshwater eel poaching and smuggling</span>

Freshwater eel poaching and smuggling have emerged in recent years as a direct response to the sustained popularity of eels as food combined with the eels' low population, endangered status, and subsequent protections. Freshwater eel are elongated fish in the Anguillidae family of ray-finned fish. The three most commonly consumed eel species are the Japanese eel, European eel, and American eel.

References

  1. ↑ Mundine, Carolus (1783). De Angillae Ovariis (in Latin). Vol. 6. De Bononiensi Scientiarum et Artium Instituto atque Academia Commentarii. 6:406–18
  2. ↑ Freud, Sigmund (1877). Beobachtungen über Gestaltung und feineren Bau der als Hoden beschriebenen Lappenorgane des Aals [Observations on the configuration and finer structure of the lobed organs in eels described as testes] (in German). Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften. Mathematisch-Naturwissenschaftliche Classe. Vol. 75, p. 419. Freud's study was in response to Szymon Syrski's book Ueber die Reproductions-Organe der Aale (1874); see Ursula Reidel-Schrewe "Freud's Début in the Sciences" in: Sander L. Gilman, Jutta Birmele, Jay Geller, Valerie D Greenberg (eds.), Reading Freud's Reading, NYU Press, 1995, pp. 1–22.
  3. ↑ "Was dachten Nazis über den Aal? | Archiv – Berliner Zeitung" (in German). Berlinonline.de. 2004-10-20. Retrieved 2013-07-16.
  4. ↑ FH. "Der Aal im Nationalsozialismus" (in German). Wno.org. Archived from the original on December 17, 2011. Retrieved 2012-01-04.
  5. ↑ "Sigmund Freud und der Aal" (in German). Kulturkurier.de. Retrieved 2013-07-16.
  6. ↑ Tesh F.W. 2003. The eel, third edition. Published by Blackwell Science. 408 pages
  7. ↑ Safran, Patrick, ed. (2009). Fisheries and aquaculture : towards sustainable aquatic living resources management. Vol. 3. Oxford: UNESCO. p. 76. ISBN   978-1-84826-560-8.
  8. ↑ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  9. ↑ "Ancient mystery of European eel migration unravelled to help combat decline of critically endangered species". Department for Environment, Food and Rural Affairs . Retrieved 15 October 2022.
  10. ↑ Wright, R.M.; Piper, A.T.; Aarestrup, K. (2023). "First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea". Scientific Reports. 12. doi: 10.1038/s41598-022-19248-8 . PMC   9562336 .
  11. ↑ Tsukamoto, Katsumi (23 February 2006). "Spawning of eels near a seamount". Nature. 439 (7079): 929. doi: 10.1038/439929a . PMID   16495988. S2CID   4346565.
  12. ↑ Chow, S.; Kurogi, H.; Mochioka, N.; Kaji, S.; Okazaki, M.; Tsukamoto, K. (2009). "Discovery of mature freshwater eels in the open ocean". Fisheries Science. 75: 257–259. doi:10.1007/s12562-008-0017-5. S2CID   39090269.
  13. ↑ Jim Cambray (April 2004). "African freshwater eels – new tools in environmental education". Science in Africa. Archived from the original on 2013-03-17. Retrieved 2013-03-29.
  14. 1 2 3 Jellyman, D.; Tsukamoto, K. (2010). "Vertical migrations may control maturation in migrating female Anguilla dieffenbachii". Marine Ecology Progress Series. 404: 241–247. Bibcode:2010MEPS..404..241J. doi: 10.3354/meps08468 .
  15. ↑ Jellyman, D. (2006). "Tagging along when longfins go spawning" (PDF). Water & Atmosphere. 14 (1): 24–25.
  16. 1 2 McDowall, R. M. (1990). New Zealand freshwater fishes: a natural history and guide (Rev. ed.). Auckland: Heinemann-Reed.
  17. ↑ Chisnall, B. L.; Jellyman, D. J.; Bonnett, M. L.; Sykes, J. R. (2002). "Spatial and temporal variability in length of glass eels (Anguilla spp.) in New Zealand". New Zealand Journal of Marine and Freshwater Research. 36 (1): 89–104. doi: 10.1080/00288330.2002.9517073 .
  18. ↑ "Demand for Baby Eels Brings High Prices and Limits". 2000-12-03. Archived from the original on December 24, 2002. Retrieved 2012-01-04.
  19. ↑ Fish (No. 2) Act 1533 (25 Hen. 8. c. 7)
  20. ↑ Salmon Fishery Act 1873 (36 & 37 Vict. c. 71)
  21. ↑ Aprahamian, Miran; Wood, Peter (February 2021). "Estimation of glass eel ( Anguilla anguilla ) exploitation in the Severn Estuary, England". Fisheries Management and Ecology. 28 (1): 65–75. doi:10.1111/fme.12455. S2CID   225134755.
  22. 1 2 Bird, Winifred. "In Japan, Captive Breeding May Help Save the Wild Eel". Yale Environment 360. Yale School of the Environment. Retrieved 28 April 2021.
  23. ↑ Schneebauer, Gabriel; Dirks, Ron P.; Pelster, Bernd (2017-08-17). "Anguillicola crassus infection affects mRNA expression levels in gas gland tissue of European yellow and silver eel". PLOS ONE. 12 (8): e0183128. Bibcode:2017PLoSO..1283128S. doi: 10.1371/journal.pone.0183128 . ISSN   1932-6203. PMC   5560681 . PMID   28817599.
  24. ↑ Würtz, J.; Taraschewski, H. (2000-01-14). "Histopathological changes in the swimbladder wall of the European eel Anguilla anguilla due to infections with Anguillicola crassus". Diseases of Aquatic Organisms. 39 (2): 121–134. doi: 10.3354/dao039121 . ISSN   0177-5103. PMID   10715817.
  25. ↑ Schofield, O.; Bergmann, T.; Bissett, P.; Grassle, J.F.; Haidvogel, D.B.; Kohut, J.; Moline, M.; Glenn, S.M. (April 2002). "The Long-term Ecosystem Observatory: an integrated coastal observatory". IEEE Journal of Oceanic Engineering. 27 (2): 146–154. doi:10.1109/JOE.2002.1002469. ISSN   0364-9059.

Sources and further reading

  • Banks, R.C., R.W. McDiarmid, A.L. Gardner, & W.C. Starnes (2003). Checklist of Vertebrates of the United States, the U.S. Territories, and Canada.
  • Bussing, W.A. (1998). Peces de las aguas continentales de Costa Rica [Freshwater fishes of Costa Rica]. 2nd ed. San José Costa Rica: Editorial de la Universidad de Costa Rica.
  • Butsch, R.S. (1939). A list of Barbadian fishes. J. B.M.H.S. 7(1): pp. 17–31.
  • Böhlke, J.E. & C.C.G. Chaplin (1993). Fishes of the Bahamas and adjacent tropical waters. 2nd edition. University of Texas Press, Austin.
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