Atlantic Salmon Habitat Model
go to: USFWS Gulf of Maine Watershed Habitat Analysis
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Draft Date:
July 2001

Atlantic salmon, Salmo salar

Use of Study Area Resources:
Reproduction, growth and migration. Freshwater habitats are used for spawning and by fry, and parr (early juveniles). Estuarine and marine habitats used by smolt (older juveniles) and adults may be extensive, but their occupation of the Gulf of Maine is transitory.

Habitat Use:
Atlantic salmon historically were a premier recreational and food fish species in New England. Over-harvest in natal rivers and the high seas, degradation and obstruction of streams, and changes in the prey base have caused a drastic population decline. This has been only partly mitigated by stocking of young fish and removal of a number of obstructions.

Danie et al. (1984) provides the following summary of salmon life history. Atlantic salmon ascend freshwater streams to spawn on gravel substrate from mid-October to mid-November. In Maine, eggs incubate for 175 to 195 days depending on water temperature, and hatch in April or early May. After hatching, the 15 mm long yolk-sac larvae (alevins), remain buried in the gravel depressions for up to 6 weeks while absorbing the yolk-sac for nourishment. The resulting 25 mm long fry begin foraging for themselves and emerge, usually at night, from the gravel depressions. Larger freshwater juveniles (parr) will remain in riffle sections of streams until they are 125-150 mm in length, which may take from 2 to 3 years. Failure to attain this length by spring or early summer of the year, will prevent parr from transforming into smolts (seaward migrating juveniles). After attaining this critical length, parr undergo smoltification which includes physical and physiological changes adaptive to a migration to a marine environment. The parr marks disappear and the skin develops a silvery pigmentation from deposition of guanine in the skin, the tail lengthens and becomes more deeply forked, and schooling behavior develops. Increases in water temperature and water level trigger downstream migration of smolts. Smolts from the western Atlantic migrate, within 3 m of the surface of the ocean, to feeding areas in the Davis Strait between Labrador and Greenland. Atlantic salmon will return to natal rivers to spawn after 1 (grilse) or 2 (bright salmon) years at sea. Salmon accumulate in estuaries, bays, and river mouths, before ascending streams. Upstream migration of salmon coincides with increases in water flow. Adult salmon do not feed while in freshwater. Atlantic salmon do not consistently die after spawning, and many spent fish (kelts) survive the winter in freshwater and begin to feed again. Mortality is high when kelts enter saltwater. Those kelts that survive and migrate to feeding grounds in the Davis Strait, may become repeat spawners.

Habitat Mapping:
Stanley and Trial 1995 produced a habitat suitability model for freshwater stages of Atlantic salmon (egg, embryo, fry, parr) based on water temperature, dissolved oxygen, pH, velocity, and depth.  Information on these parameters is not available for most rivers in the Gulf of Maine, and so we mapped freshwater habitat from occurrence information.

Riverine habitat was 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 in the 1:100,000 maps. 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 ( We also integrated a coverage developed by New Hampshire Department of Environmental Services, using information provided by New Hampshire Game and Fish.  In Maine we utilized a GIS coverage based on detailed habitat surveys conducted between 1986 and 2000 by the Maine Atlantic Salmon Commission and USFWS.  Information was also obtained from the USFWS/US Department of Commerce Status Review, which listed New England streams and rivers having current or recent salmon usage, supplemented with comments from Maine Atlantic Salmon Commission biologists.

Artifacts of our grid-cell mapping of riverine themes may in some cases have caused habitat omissions, while mapping of streams sometimes indicated aquatic habitat values in primarily upland areas. The former was because fish habitats were gridded from continuous polygons/arcs derived from USGS hydrology coverages. Conversion of polygons to grids 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. Single line arcs (streams) 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.  

Coastal habitat was mapped at two levels.  Inshore habitat for juveniles and adults was based on the NOAA National Marine Fisheries Service designations constituting "Essential Fish Habitat" for Atlantic salmon.  These are estuaries and embayments generally at the mouths of important salmon rivers (NEFMC EFH Amendment Oct. 7, 1998, downloaded from, on 4/5/01).  The Essential Fish Habitat boundaries were based on salinity zones, obtained from NOAA NOS (, downloaded 4/5/01), and adjusted to fit our higher resolution coastline data.  Because juvenile and adult salmon may range throughout open waters of the Gulf of Maine, all other coastal areas were mapped as habitat, but assigned a lower value than these specific inshore areas.

Habitat Suitability:
Habitat suitability for this model primarily is scored on the basis of our knowledge of current use by Atlantic salmon.  Distinctions in scoring also allow the source information to be distinguished in map products. Rivers known to be used were scored 1.0; those apparently offering accessible habitat but not recently surveyed were scored 0.5. Inshore areas at the mouths of salmon rivers (Atlantic salmon coastal Essential Fish Habitats) were scored 0.4 .  The balance of the Gulf of Maine, having depths of mid-intertidal or deeper, were scored 0.2.

Danie, D.S., J.G. Trial and J.G. Stanley. 1984. Species profiles: life histories and environmental requirements of coastal fish and invertebrates (North Atlantic) Atlantic salmon. U.S. Fish and Wildlife Service, FWS/OBS-82/11.22. 19 pp.

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.

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.

Stanley, J.G. and J.G. Trial. 1995. Habitat suitability index models: nonmigratory freshwater life stages of Atlantic salmon. USDI/NBS Report 3. 19 pp.