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Like the animals they seek, members of the VIMS' American Eel Monitoring Survey team team spend a lot of spring days struggling through muck and climbing over obstacles.
Their quest is to count the young eels now migrating into Virginia's estuaries and freshwater lakes and ponds.
The team began their annual survey of juvenile eels during spring 2000, in response to a 1998 mandate from the Atlantic States Marine Fisheries Commission (ASMFC). The mandate calls for Atlantic coast states to monitor the recruitment of juvenile eels from the ocean into estuaries. The ASMFC issued the mandate due to recent declines in landings of adult American eels along the Atlantic coast. Those declines are clearly evident in data collected by the VIMS Trawl Survey Team, which has been recording the abundance of adult eels in Virginia's waters since 1955.
American eels are catadromous - meaning they live their life in freshwater but breed in the ocean. They thus reverse the life cycle of anadromous fishes such as striped bass and salmon that live in the ocean but return to freshwater to spawn.
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| Life cycle of the American eel entering Virgina waters. Image courtesy of AEMS. |
The eel's life cycle is complex. Immature eels inhabit fresh and brackish coastal waters from North America to Greenland and range inland as far as the Mississippi and Great Lakes drainages. They spawn during the winter to early spring in the Sargasso Sea - a vast eddy in the Atlantic Ocean near Bermuda. After spawning the adults die. The eggs hatch into leaf-shaped larvae called leptocephali, which are transported by ocean currents in a northwesterly direction. The young metamorphose into transparent "glass" eels near the western Atlantic coast. Coastal currents and active migration transport the glass eels into Virginia's rivers and estuaries from March to June.
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| Leptocephali larva. Image courtesy of ecoSCOPE. |
As glass eels migrate upriver into freshwater ponds and lakes, they grow larger and become pigmented. Young pigmented eels less than six inches long are known as "elvers." As they continue to grow, elvers become "yellow eels" - the resident immature individuals typically caught by the Chesapeake's recreational anglers and commercial watermen. Yellow eels are active at night and eat most anything, including insects, mollusks, crustaceans, worms, and other fish. They can grow up to five feet long.
Yellow eels inhabit the fresh and brackish waters of the Bay until they reach sexual maturity. Age at maturity varies greatly with location. Eels in southern locations mature at an earlier age then their northern counterparts. Maturation in the Chesapeake Bay ranges from 8 to 24 years.
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| Glass eel. Image courtesy of ecoSCOPE. |
Each fall, eels that have reached sexual maturity begin migrating to their ocean spawning grounds, which lie at a depth of 2000 meters (~ 6,000 feet) beneath the Sargasso Sea. Sexual maturation involves a final metamorphosis that prepares the eels for ocean life. Their skin becomes thicker, and changes from yellow to a bronze black sheen. Their bodies grow fatter, their pectoral fins enlarge, and their eyes grow larger and change pigmentation. Their swim bladders also change, and their digestive systems disintegrate, so they no longer eat.
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| Photo of yellow eel by Garold W. Sneegas courtesy General College and James Ford Bell Museum of Natural History, University of Minnesota |
Since the Chesapeake Bay states account for nearly 90% of U.S. commercial landings, both Virginia and Maryland have taken an active and intense role in eel research. The VIMS team surveys the James, York, Rappahannock, and Potomac Rivers for the juvenile eels. For example, the York River sites are sampled daily throughout the eel run. Currently this work is funded through the Virginia Marine Resources Commission Commercial and Recreational Fishery Advisory Boards and the Potomac River Fisheries Commission.
The harvesting of eels during the glass or elver stage is illegal in most states including Virginia. However, illegal poaching does take place with most of the catch shipped oversees to Europe and Japan.
Technical reports by VIMS' American Eel Monitoring Survey team
Links to eel information: