American Eel Habitat Model
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American eel, Anguilla rostrata
Use of Study Area Resources:
Migration and growth of elvers/juveniles (adult American eels spawn in the 'Sargasso Sea', in the mid-Atlantic Ocean). American eels are found from Greenland southward to the east coast of South America (Atlantic States Marine Fisheries Commission 2000).
American eels have "multiple habitat requirements, utilizing open oceans, large coastal tributaries, small freshwater streams, lakes and ponds" (Atlantic States Marine Fisheries Commission 2000). Eggs hatch at sea. The larval stage (leptocephalus) lasts about a year, during which time they are transported along the eastern coast of the U.S. by the Antilles Current, Florida Current, and eventually the Gulf Stream. Gradually the leptocephalus develops into an eel-shaped juvenile form, of which the initial unpigmented stage is known as the glass eel. As these reach a length of 6-9 cm they turn brown and are known as elvers. Elvers actively migrate toward estuaries and freshwater in the spring, most moving up rivers and streams over a period of months or even years. (Ross 1991) They can survive out of water so long as their skin is moist, and this may contribute to their ability to surmount stream obstructions including dams and falls that block migration of other species (Bigelow and Schroeder 1953, Ross 1991). Because of their small size, elvers are unable to ascend fish ladders designed for adult anadromous species; they will attempt to scale wet surfaces include dam faces and wet rocks (Atlantic States Marine Fisheries Commission 2000). Some elvers also reside in coastal embayments and nearshore areas. (Atlantic States Marine Fisheries Commission 2000).
As elvers grow, they enter a sub-adult stage known as yellow eels. Growth towards maturation occurs over a period of 6-7 years in the Chesapeake region (Hedgepeth 1983 in Atlantic States Marine Fisheries Commission 2000), and is believed to occur more slowly in colder northern waters. During this time yellow eels are bottom dwellers, feeding opportunistically on whatever food is available, living or dead, including frogs, crustaceans, insects, worms, shellfish, and small fish (Bigelow & Schroeder 1953, Ross 1991). Eels are primarily nocturnal foragers, returning to specific resting areas by day, or burrowing into soft mud leaving only their heads protruding (Ross 1991, Fahey 1978 in Atlantic States Marine Fisheries Commission 2000). The skin color of yellow eels varies widely, depending on the substrate, and can lighten or darken over a period of hours if they are moved to a different bottom. Typically they are muddy brown or olive brown above, with yellow-brown sides, and a yellowish-white belly. Females grow to larger sizes than males, reaching lengths from 2 to 4 feet. (Bigelow and Schneider 1953). Important habitats for yellow eels include the soft, undisturbed sediments of river and lake bottoms, estuaries, and large streams.
With the onset of sexual maturity, eels undergo a final metamorphosis into the silver eel stage. During this stage, which occurs in Autumn, the skin thickens and darkens to a metallic bronze color, the body fattens and eyes grow more prominent. The maturing eels descend the streams by night, no longer feeding. Migrating eels are highly capable of passing over, around and under stream barriers. The digestive tract degenerates, and maturation of the gonads occurs after they reach salt water. Spawning occurs midwinter in the Sargasso Sea of the Atlantic, and death follows. (Bigelow and Schroeder 1953, Ross 1991)
Eels residing in coastal habitats may use a wide range of substrate types,
temperatures, flows, and salinity levels. They are found to be most
prevalent in the nearshore, shallow embayments and tributaries (Atlantic
States Marine Fisheries Commission 2000). Adult oceanic habitat
requirements are not known. However, American eel have been taken at depths
greater than 6000 meters (Atlantic States Marine Fisheries Commission 2000).
Riverine and lake habitat was mapped primarily from occurrence data, obtained from several sources. Eipper et al. (1982) mapped upstream migratory pathways for catadromous and anadromous fishes throughout New England at a relatively small scale. We coded the corresponding stream segments as suitable habitat on 1:24,000 USGS digital maps (Maine, New Hampshire) and 1:100,000 maps (Massachusetts), supplemented by 1:24,000 for minor features absent in the 1:100,000 maps. Where eel migration pathways were not specifically labeled, we assumed that they could use streams and ponds frequented by other migratory species. Eipper was used for the whole study area. This general information was considerably supplemented state by state with more specific surveys. In Massachusetts we were supplied with point data from Hartel et al. (in press), and from a GIS coverage developed by Massachusetts Department of Fisheries, Wildlife and Environmental Law Enforcement (http://www.state.ma.us/dfwele/gisprog/gisanad.htm). In New Hampshire we added migratory pathways that New Hampshire Fish and Game biologists informed us were used by other anadromous species. In Maine we used Maine Department of Inland Fisheries and Wildlife unpublished stream electro-fishing and other stream survey information and a state-wide fisheries survey of lakes and ponds. We coded as habitat all reaches downstream of ponds having eels. We also coded all tributaries of the Pleasant, Sheepscot, Medomak, and East Machias river as eel habitat on the basis of observations by Ken Oliveira (Univ. of Maine). Occurrences of catadromous or anadromous species at a sample site were taken as indicating use of all downstream reaches.
Artifacts of grid-cell mapping of stream and riverine themes may in some cases cause habitat omissions, and in others aquatic habitat values in primarily upland areas. Fish habitats were gridded from continuous polygons/arcs derived from USGS hydrology coverages. Single line arcs were necessarily converted into strings of cells, each with a minimum width of 30 m (the cell dimensions). Even where the dominant land cover of a cell was upland, we retained the habitat value for a stream passing through the cell. As a result, some upland areas will display habitat value for anadromous fishes.
The grid process left discontinuities when converting narrow (< 30 m wide) polygon features, such as small rivers, where only part of a cell was crossed by aquatic habitat. Moreover, where National Wetlands Inventory polygons representing wetlands and water bodies differed from those of USGS, the only fish habitat we retained was that which corresponded to aquatic classes in the former. Therefore, fish habitats and migratory pathways may not appear as continuous extents of habitat, or extend to the boundaries of contiguous wetlands in all cases.
Coastal habitat was mapped at two levels. Inshore areas which
may serve as habitat for juveniles and adults ("harbor, stream mouth, muddy
estuary or tidal marsh" of Bigelow and Schroeder 1953), were mapped by selecting
estuarine and marine portions of the study area having unconsolidated substrates
and depths from mid-intertidal down to 60'. Deeper areas, which may
be used during migration by larvae and migrating adults were mapped
as lower value habitat.
Habitat suitability for this model is scored on the basis of known or apparent use. Riverine and lake habitats delineated from known occurrences were scored 1.0; inshore areas were scored 0.5, and areas deeper than 60' were scored 0.2.
Bigelow, H.B. and W.C. Schroeder. 1953. Fishes of the Gulf of Maine. U.S. Fish. Wildl. Serv. Fish. Bull. 74(53), 577 p.
Eipper, A., W. Knapp and C. Laffin. 1982. Anadromous fish streams of New England: upstream migratory routes. Portfolio NE-1. USDI Fish and Wildlife Service.
Atlantic States Marine Fisheries Commission. 2000. Fishery Management Report No. 36. Interstate Fishery Management Plan for American eel (Anguilla rostrata). 79 pp. http://www.asmfc.org/PUB/FMRs/American%20Eel%20FMP.pdf (available 4/2001)
Hartel, K.E., D.B. Halliwell and A.E. Launer. (in press) Inland Fishes of Massachusetts. Natural Hist. New England Series. Massachusetts Audubon Society, Lincoln, MA.
Ross, M.R. 1991 Recreational Fisheries of Coastal New England. University of Massachusetts Press, Amherst, MA. 279 pp.