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COSEWIC assessment and status report on the Atlantic Salmon (Lake Ontario population) in Canada

Habitat

Habitat requirements

Elson (1975) described a good Atlantic salmon river as having clean water seldom rising above about 25°C with a general gradient of about 2-11.5 m/km.  The river should have a naturally graded, stable bed with a stony bottom comprised of particles varying in size from coarse sand and gravel to large boulders. However, Atlantic salmon have specific habitat requirements at each stage of their life history.

Adult Atlantic salmon build redds, or nests, and spawn in areas of shallow (20-30 cm) and swift-running water (40-50 cm·s-1), mainly located near the banks of the river where the flow of the water is broken (Bardonnet & Baglinière 2000; Moir et al. 1998).  The substrate is made up of a small proportion of sand and a large proportion of coarse materials ranging from gravel to cobble (Bardonnet & Baglinière, 2000).  These areas are also characterized by low (0.2%) to moderate (1%) gradients (Elson 1975).

Lake Ontario salmon in particular were observed to spawn on gravel shoals in clear, cold streams with rather steep gradients (Parsons 1973).  Conditions appropriate for such spawning grounds are typically found at the upstream side of riffles or gravel bars where the concentration of fines is low (10-15% by weight) and permeability is high (>900 cm·h-1) and thus dissolved oxygen levels are high (Peterson 1978; cited in Fleming 1998).  Spawning runs, in Lake Ontario tributaries, typically began in October (Goodyear et al. 1982) and spawning typically began in mid-November; but an April/May run was known to occur in streams west of Toronto (Huntsman 1944).

Alevins emerge from the egg and they usually remain near the incubation area (unless taken downstream as drift) until the absorption of the yolk sac is complete.  As fry, the salmon prefer habitat similar to the spawning areas and may remain there or disperse to similar habitat (riffles with coarse substrate) within 200 m downstream of the redd (Beall et al. 1994).

Juvenile Habitat

Atlantic salmon parr, ages 1-3 years, settle in riffles characterized by cobble substrate, water velocities of 10-60 cm·s-1, and shallow to intermediate water depth (20-70 cm) (Heggenes & Salveit 1990).  In Lake Ontario tributaries in particular, juvenile Atlantic salmon have been found to prefer areas above barriers where stream gradients are typically steeper and rock substrate is more abundant (Stanfield and Jones 2003).

Juvenile Atlantic salmon are typically benthic drift feeders and rely on the swift currents to supply a continuous forage base (Wankowski & Thorpe 1979).  Rimmer et al. (1984) and Cunjak (1996) demonstrated an autumnal shift to deeper stream areas by juveniles.  The over-wintering habitats are mainly characterized by suitably sized, unembedded rock shelters (>15-20 cm in diameter) (Rimmer et al. 1984).  The size of the “home stone” is usually correlated with the size of the parr (Cunjak 1988).

Adult Habitat

‘Lake’ salmon typically remain in their lake until immediately prior to spawning at which time they ascend into their natal stream and establish a spawning site.  The small size of most tributaries of Lake Ontario and their low flow and volume were, in most cases, unfavourable for the extended residency of large salmon (Parsons 1973). Adults rarely remained in the streams longer than 1-week post spawn; none were present in the streams in December (Parsons 1973).  Little is known about the preferred lacustrine habitat of Atlantic salmon except that deep, cool, oligotrophic conditions within the host lake, a forage base that includes rainbow smelt (Osmerus mordax), and the presence of feeder streams providing suitable spawning and nursery habitat, appear to be the most ecologically suitable (MacCrimmon & Gots 1979, Cuerrier 1983).  With a surface area of 18,960 km2, an average depth of 86 m (Lake Ontario LaMP 2004), and at the time when Atlantic salmon were plentiful, and there were few competitors for food, Lake Ontario may have served the same function for adult and juvenile lake salmon as the ocean did for anadromous populations.

 

Trends

Recently, there have been significant efforts to restore and enhance the habitat in and around traditional salmon spawning streams, particularly in riparian areas.  The removal and failure of numerous dams have increased access to more suitable spawning areas, and improved the potential for stocked Atlantic salmon to survive and breed in Lake Ontario tributaries.  It is important to note that the continued increase in urbanization (and associated increase in percent impervious cover) of the Greater Toronto Area is likely to have direct and indirect impacts on the physio-chemical and biological characteristics of streams within associated watersheds (Stanfield and Kilgour 2005, Stanfield et al. 2005). 

Within the lake itself, there have also been many changes that may impact Atlantic salmon suvival including the introduction of Pacific salmon and other non-native salmonine species (Christie 1973), and the invasion of Lake Ontario by sea lamprey (Christie 1972) and zebra mussels.