American Shad Habitat Model
go to: USFWS Gulf of Maine Watershed Habitat Analysis
go to: Species Table
Feedback: We welcome your suggestions on improving this model!

Draft Date:
June 2001

The American shad, Alosa sapidissima, is an anadromous clupeid fish, ranging from Florida to the St. Lawrence River (Bigelow and Schroeder 1953). Shad are sought in both recreational and commercial fisheries.

Use of Study Area Resources:
Shad ascend tributaries and spawn in slow-flowing freshwater sections of rivers in spring and early summer (Scott and Scott 1988). Shad eggs are pelagic and are carried downstream by the current (Weiss-Glanz et al. 1988). Larvae and early juveniles use natal rivers during summer; decreasing water temperature is the stimulus for downstream movement of juveniles into brackish water and finally to the sea (Weiss-Glanz et al. 1988).  Shad mature in 3 to 6 years, and may live up to 11 years, making repeated spawning runs (Weiss-Glanz et al. 1988). Shad make extensive oceanic migrations (up to 3000 km in one season), at least partly associated with bottom water temperature (Weiss-Glanz et al. 1988, Scott and Scott 1988).  Canada Department of Fisheries and Oceans scientists (www page) found that, "rather than stopping in the Gulf of Maine as was originally thought, the main body of ocean-migrating shad move either into the upper Bay of Fundy or along the eastern coast of Canada into the mouth of the St. Lawrence River, and as far north as Nain, Labrador." Most shad move out of the Gulf of Maine in the fall, as water temperatures decline (Stier and Crance 1985).

Late juvenile and adult shad are widespread in the Gulf of Maine (NMFS trawl data, 1982 through 1999, provided by Holly Yachmetz, NOAA, Woods Hole MA).  Neves and Drepres (1979 in Stier and Crance 1985) observed that shad were found most frequently in 50 to 100 m depths. Canada Department of Fisheries and Oceans (www page) states that, in clear ocean water, shad usually remain at depths of 100 to 200 m, based on their preferred level of light intensity. The Gulf of Maine NMFS trawl data collected shad at depths of 13 to 306 m.

Habitat Mapping:
Freshwater habitats were mapped from occurrence data obtained from several sources. Eipper et al. (1982) mapped upstream migratory pathways for anadromous and catadromous 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 from the 1:100,000 data. Eipper et al. (1982) was used for the whole study area. This general information was considerably supplemented state by state with collection data. 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 ( In New Hampshire we added migratory pathways based on information from New Hampshire Fish and Game. In Maine we used Maine Department of Inland Fisheries and Wildlife unpublished stream electro-fishing and other survey information (available for southern Maine) and information from DMR biologists.

Artifacts of grid-cell mapping of 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.

Marine habitats were mapped by selecting estuarine and marine portions of the study area having depths from mid-intertidal (suitable for juveniles) down to 200 m (656').  Deeper areas were mapped as lower value habitat.

Habitat Suitability:
Habitat suitability for this model is scored on the basis of known use. All freshwater areas mapped from occurrence information were scored 1.0; marine areas within the depth zone having most apparent use were scored 0.5, and deeper areas 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.

Canada Department of Fisheries and Oceans. Ocean Migration (shad). downloaded 6/21/01

Eipper, A., W. Knapp and C. Laffin. 1982. Anadromous fish streams of New England: upstream migratory routes. Portfolio NE-1. U.S. Fish and Wildlife Service.

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.

Scott, W.B. and M.G. Scott. 1988. Atlantic fishes of Canada. Can. Bull. Fish. Aquat. Sci. 219:731.

Stier, D.J. and J.H. Crance. 1985. Habitat suitability index models and instream flow suitability curves: American shad. USFWS Biol. Report 82(10.88). 34pp.

Weiss-Glanz, L.S., J.G. Stanley, J.R. Moring,and D. Moran. 1986. Species profiles: life histories and environmental requirements of coastal fishes and invertebrates (North Atlantic), American shad. USFWS Biol. Report 82(11.59).