Eel life history
Past studies of eels
The European eel (Anguilla anguilla) was historically 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 nature.
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 such 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
European eel
The
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 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, crossing wet grasslands at night to reach rivers that lead to the sea. Eel migration 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.
How the adults make the 6,000 km (3,700 mi) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remains unknown. By the time they leave the continent, 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
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
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.
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
Southern African eels
Southern Africa's four species of freshwater eels (
New Zealand longfin eels
Decline of the glass eels
No one yet knows the reasons, but 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.
In 1997,
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 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.[20][21]
The demand for adult eels has continued to grow, as of 2003[update]. 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 detrimental 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
Threats to eels
Strong concerns exist that the European eel population might be devastated by a new threat: 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
In
See also
- Eel ladder
- Fish migration
References
- ^ Mundine, Carolus (1783). De Angillae Ovariis (in Latin). Vol. 6. De Bononiensi Scientiarum et Artium Instituto atque Academia Commentarii. 6:406–18
- ^ 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.
- ^ "Was dachten Nazis über den Aal? | Archiv – Berliner Zeitung" (in German). Berlinonline.de. 2004-10-20. Retrieved 2013-07-16.
- ^ FH. "Der Aal im Nationalsozialismus" (in German). Wno.org. Archived from the original on December 17, 2011. Retrieved 2012-01-04.
- ^ "Sigmund Freud und der Aal" (in German). Kulturkurier.de. Retrieved 2013-07-16.
- ^ Tesh F.W. 2003. The eel, third edition. Published by Blackwell Science. 408 pages
- ISBN 978-1-84826-560-8.
- Greenwood Press.
- ^ "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.
- .
- S2CID 4346565.
- S2CID 39090269.
- ^ 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.
- ^ .
- ^ Jellyman, D. (2006). "Tagging along when longfins go spawning" (PDF). Water & Atmosphere. 14 (1): 24–25.
- ^ a b McDowall, R. M. (1990). New Zealand freshwater fishes: a natural history and guide (Rev. ed.). Auckland: Heinemann-Reed.
- .
- ^ "Demand for Baby Eels Brings High Prices and Limits". 2000-12-03. Archived from the original on December 24, 2002. Retrieved 2012-01-04.
- ^ 25 Henry VIII, c. 7
- ^ 36 & 37 Vict c. 71
- S2CID 225134755.
- ^ a b Bird, Winifred. "In Japan, Captive Breeding May Help Save the Wild Eel". Yale Environment 360. Yale School of the Environment. Retrieved 28 April 2021.
- PMID 28817599.
- PMID 10715817.
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.
- Claro, R. (1994). Characterísticas generales de la ictiofauna. pp. 55–70. [In] R. Claro [ed.] Ecología de los peces marinos de Cuba. Instituto de Oceanología Academia de Ciencias de Cuba and Centro de Investigaciones de Quintana Roo.
- Claro, Rodolfo, & Lynne R. Parenti (2001). Chapter 2: The Marine Ichthyofauna of Cuba. [In] Claro, Rodolfo, Kenyon C. Lindeman, & L.R. Parenti, [eds.] Ecology of the Marine Fishes of Cuba. Smithsonian Institution Press. Washington, DC, USA. pp. 21–57. ISBN 1-56098-985-8.
- Erdman, D.S. (1984). Exotic fishes in Puerto Rico. pp. 162–176. [In] W.R. Courtney, Jr. & J.R. Stauffer, Jr. [eds.] Distribution, biology and management of exotic fishes. Johns Hopkins University Press, Baltimore, USA.
- Eschmeyer, William N., [ed.] (1998). Catalog of Fishes. Special Publication of the Center for Biodiversity Research and Information, no. 1, vol 1–3. California Academy of Sciences. San Francisco, California, USA. 2905. ISBN 0-940228-47-5.
- Fish, M.P. & W.H. Mowbray (1970). Sounds of Western North Atlantic fishes. A reference file of biological underwater sounds. The Johns Hopkins Press, Baltimore, MD.
- Food and Agriculture Organization (1992). FAO yearbook 1990. Fishery statistics. Catches and landings. FAO Fish. Ser. (38). FAO Stat. Ser. 70:(105)
- Food and Agriculture Organization (1997). Aquaculture production statistics 1986–1995. FAO Fish. Circ. 815, Rev. 9.
- Greenfield, D.W & J.E Thomerson (1997). Fishes of the continental waters of Belize. University Press of Florida, Florida.
- International Game Fish Association (1991). World record game fishes. International Game Fish Association, Florida, USA.
- Jessop, B.M. (1987). Migrating American eels in Nova Scotia. Trans. Amer. Fish. Soc. 116: pp. 161–170.
- Kenny, J.S. (1995). Views from the Bridge: A memoir on the freshwater fishes of Trinidad. Julian S. Kenny, Maracas, St. Joseph, Trinidad, & Tobago.
- Lim, P., Meunier, F.J., Keith, P. & Noël, P.Y. (2002). Atlas des poissons et des crustacés d'eau douce de la Martinique. Patrimoines Naturels, 51: Paris: MNHN.
- Murdy, Edward O., Ray S. Birdsong, & John A. Musick 1997. Fishes of Chesapeake Bay. Smithsonian Institution Press. Washington, DC, USA. ISBN 1-56098-638-7.
- Nelson, Joseph S., Edwin J. Crossman, Héctor Espinosa-Pérez, Lloyd T. Findley, Carter R. Gilbert, Robert N. Lea, & James D. Williams, [eds.] (2004). Common and scientific names of fishes from the United States, Canada, and Mexico, Sixth Edition. American Fisheries Society Special Publication, no. 29. American Fisheries Society. Bethesda, Maryland, USA. ISBN 1-888569-61-1.
- Nielsen, J.G. and E. Bertelsen (1992). Fisk i grønlandske farvande. Atuakkiorfik, Nuuk. 65 s.
- Nigrelli, R.F. (1959). Longevity of fishes in captivity, with special reference to those kept in the New York Aquarium. pp. 212–230. [In] G.E.W. Wolstehnolmen & M. O'Connor [eds.] Ciba Foundation Colloquium on Ageing: the life span of animals. Vol. 5., Churchill, London.
- Ogden, J.C., J.A. Yntema, & I. Clavijo (1975). An annotated list of the fishes of St. Croix, U.S. Virgin Islands. Spec. Publ. No. 3.
- Page, L.M. & B.M. Burr (1991). A field guide to freshwater fishes of North America north of Mexico. Houghton Mifflin Company, Boston.
- Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Robins, C.R. & G.C. Ray (1986). A field guide to Atlantic coast fishes of North America. Houghton Mifflin Company, Boston, MA
- Robins, Richard C., Reeve M. Bailey, Carl E. Bond, James R. Brooker, Ernest A. Lachner, et al. (1980). A List of Common and Scientific Names of Fishes from the United States and Canada, Fourth Edition. American Fisheries Society Special Publication, no. 12. American Fisheries Society. Bethesda, MD.
- Robins, Richard C., Reeve M. Bailey, Carl E. Bond, James R. Brooker, Ernest A. Lachner, et al. 1980. A List of Common and Scientific Names of Fishes from the United States and Canada, Fourth Edition. American Fisheries Society Special Publication, no. 12. American Fisheries Society. Bethesda, MD.
- Smith, C.L. (1997). National Audubon Society field guide to tropical marine fishes of the Caribbean, the Gulf of Mexico, Florida, the Bahamas, and Bermuda. Alfred A. Knopf, Inc., New York, NY.
- Tesch, F.-W. (2003) The eel. Blackwell Science, Oxford, UK.
- Wallace, Karen (1993) Think of an Eel, Walker Books, UK. [A picture book for children that describes the life cycle of the eel.]
- Wenner, C.A. (1978). Anguillidae. [In] W. Fischer [ed.] FAO species identification sheets for fishery purposes. West Atlantic (Fishing Area 31). volume 1. FAO, Rome, IT.
External links
- The Maine Eel and Elver Fishery, Maine Department of Marine Resources
- The Maine Eel and Elver Fishery, archived copy
- Fishbase entry for Anguilla anguilla
- Fishbase entry for Anguilla rostrata
- ICES report about eel stock collapse
- U.K Glass Eels — a large commercial firm's website, with history and fact pages
- Projekt eelBASE