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Habitat Requirements

General habitat characteristics for Atlantic salmon can be divided into those areas used for spawning, rearing, and migrating in fresh water and those marine areas used during migration and feeding. 

Adult salmon can arrive at spawning grounds several months prior to actual spawning and seek holding areas that provide shade, protection from predators, and protection from environmental variables such as high flows, high temperatures, and sedimentation.  Adults most frequently spawn from October through mid-November (Baum 1997) in areas with coarse gravel or rubble substrate and adequate cool water to keep eggs buried and well oxygenated (Peterson 1978).  Spawning is usually initiated as water temperatures drop to 10.5°C (51°F) and ceases below 7°C (45°F).  The depth of the water at spawning areas averages 36 cm and water velocities average 49 cm/second (cm/sec) (Jordan and Beland 1981). 

About 80 percent of the parr in Maine streams remain in fresh water for 2 years, while the rest remain for a third year (Baum 1997).  Thus, stream habitat must provide extensive nursery areas for young salmon to find sufficient food and protection during this stage of life (Baum 1997).  Parr can disperse upstream or downstream from the spawning area to find additional rearing habitat and preferred water temperatures (16°C, Beland 1984).  Upper lethal limits for Atlantic salmon are 33°C (Elliott 1991).

In terms of habitat availability, the table below shows current estimates.

Estimates of habitats within each SHRU that are both suitable and accessible:


Total Estimated Habitat Units By SHRU[1]



Estimated Habitats that are Suitable and Accessible

Penobscot Bay




6,820  (2% of total HU)

Merrymeeting Bay  




7,035  (2% of total HU)





23,316  (39% of total HU)





37,171  (5% of total HU)

[1] These numbers reflect a work in progress and efforts are underway to update them as of April, 2015.

Little is known about the marine habitat characteristics of Atlantic salmon.  Smolt movement in the predominantly freshwater portions of the estuary are thought to be passive, moving outward (seaward) or inward (shoreward) with the tide, and becoming more outward-moving as they  transition into more saline water (Lacroix and McCurdy 1996).  After completing the physiological transition to salt water, the post smolts grow rapidly and move in schools and loose aggregations close to the surface (Dutil and Coutu 1988, Kocik et al. 2009).  Decreasing nearshore temperatures in autumn appear to trigger offshore movements (Dutil and Coutu 1988).  The 1SW and multi-sea-winter Atlantic salmon are thought to behave similarly to post smolts, moving through the top 3 meters (m) of the water column (Reddin 1985).

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