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



Lake Ontario Atlantic salmon appeared in the spawning tributaries as early as March and early April; whereas Gulf of St Lawrence salmon do not begin their migration before May, which agrees with the timing of warming of the water there (Huntsman 1944).

The female Atlantic salmon selects a nesting site in a gravel-bottom riffle area and then uses her caudal fin to dig a nesting depression or redd (Fleming 1998).  The eggs and sperm are deposited in the redd and then covered with gravel by the female.  This activity is repeated, including redd creation, until spawning is completed (Fleming 1998). Atlantic salmon are iteroparous; spawning repeatedly during their lifetime.  The number of eggs varies from population to population, but the average female deposits approximately 700 eggs/pound (~1540 eggs/kg) of body weight (Scott and Crossman 1973).  Following spawning, any surviving adult fish or kelt, drift downstream.   They begin feeding and may return the following or subsequent years to spawn again.

Life cycle of the Atlantic salmon courtesy of the Atlantic Salmon Federation. Original watercolour by J.O. Pennanen

Figure 5.  Life cycle of the Atlantic salmon courtesy of the Atlantic Salmon Federation. Original watercolour by J.O. Pennanen.



Atlantic salmon, like all other bony fishes, are ectothermic and so are dependent upon the surrounding water temperature to cue migratory patterns, to drive metabolic processes, and to determine the rate and success of progression from one life stage to the next (Dymond 1963; Elson 1975; Wilzbach et al. 1998).  Water temperature is an important factor in river ascent by returning adults, together with river discharge (Banks, 1969).  Dependent upon the location of the population, adult salmon ascend spawning streams following afternoon temperature maxima between 16°C and 26°C (Elson 1975).  Optimum temperature for egg fertilization and incubation is approximately 6°C (MacCrimmon & Gots 1979).  Most juvenile growth has been found to occur at temperatures above 7°C (Elson 1975).  The preferred or optimal summer stream temperature for the growth and survivorship of Atlantic salmon is 17°C (Javoid & Anderson 1967), while the upper incipient lethal temperature for Atlantic salmon is 27.8°C (Garside 1973); however, adult and juvenile salmon may live for short periods above the ultimate lethal temperature (Fry 1947).  A sudden increase in incipient temperature of 10°C may bring about the death of resident salmon at temperatures considerably below the upper lethal temperature (MacCrimmon & Gots 1979).

Atlantic salmon juveniles undergo a series of changes at approximately 2-3 years of age and at a critical body length (varies according to location and population) which leads to outmigration (McCormick et al. 1998).   Behavioural changes include: loss of positive rheotactic behaviour and territoriality, adoption of a downstream orientation, and schooling tendencies (McCormick et al. 1998).  The outmigrating period is a critical stage for imprinting to odours used for homing (McCormick et al. 1998).  The transition is cued by photoperiod and temperature, while temperature and water flow appear to be key factors regulating the timing of downstream movements (McCormick et al. 1998).



When juvenile Atlantic salmon reach a certain size-related developmental stage in freshwater, they leave their rearing tributaries in spring and head downriver, to marine or lacustrine feeding areas (McCormick et al. 1998).  Although growth opportunities are generally better at sea than in fresh water, and mortality may be higher in the marine environment, the “decision” as to whether greater advantage is gained from migration or residence depends on the balance of these two factors (Gross et al. 1988).  For Lake Ontario salmon, the availability of prey in the lake may have rendered the need to travel the approximately 2400 kilometres back to the Atlantic Ocean unnecessary.

Historically, there was controversy as to whether Lake Ontario Atlantic salmon made the lengthy migration to the sea or whether they were permanent freshwater residents.  Wilmot alluded to the fact that there might be both an anadromous form and a lake-resident form of Atlantic salmon in Lake Ontario (in Parsons 1973).  In 1938, A. A. Blair published results of a comparison of scales from museum specimens of Lake Ontario Atlantic salmon with those of i) an ouananiche specimen form Lake St. John and ii) a sea salmon from Miramichi Bay, N.B.  Based on the character of the growth of these fish (measured average widths of 10 scale ridges formed during the first summer), Blair concluded that: “ the difference between the ouananiche salmon from Lake St. John and the Lake Ontario salmon is not significant but the difference between Lake Ontario salmon and the sea salmon from Miramichi Bay is significant.”  This study provided evidence that some individual Lake Ontario Atlantic salmon were non-anadromous forms.

Atlantic salmon remain in their natal stream for 2 to 3 years post hatch at which time they migrate to the sea or lake (depending upon whether the population is anadromous or non-anadromous).  The fish mature in the larger body of water and migrate back to their natal stream to spawn following 1 or 2 years of feeding and growth at sea or in the lake.  The Lake Ontario population was thought to ascend the streams in preparation for spawning in two separate runs, one in early spring (almost immediately following ice out) and another in September and October (Dymond 1965).  East of Toronto, salmon entered streams in the fall during the spawning season in October and November with exceptionally few entering in late September; both the Humber and the Credit rivers are stated to have had both spring and fall runs (Huntsman 1944).  Atlantic salmon in Lake Ontario may have dispersed throughout the lake.  Regardless of the locations, mature adults could be expected to have returned to their natal streams to spawn. 


Nutrition and feeding behaviour

Stream-dwelling juvenile salmonids are visual predators that forage by holding station in the water current within a defended territory and darting out to intercept prey (Wankowski 1981, Tucker and Rasmussen 1999).  Juvenile Atlantic salmon feed mainly upon aquatic insect larvae including chironomids, mayflies, caddisflies, blackflies and stoneflies that are supplied by the current (Scott & Crossman 1973).  Upon entry to the marine or lake environment, smolts feed opportunistically, primarily near the surface.  Their diet includes invertebrates, amphipods, euphausiids and fish (Hislop & Youngson 1984).  As post-smolts grow to adulthood in a marine environment, fish such as capelin (Mallotus villosus) and herring (Clupea harengus) become an increasingly dominant component of their diet (Mills 1989).  In Lake Ontario, the primary forage fish currently available to the Atlantic salmon are two non-native species: alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax).  Adults cease feeding when they begin their migration to the spawning grounds and consequently are in declining physiological condition at the time of spawning.

Lake Ontario salmon were known to have growth rates more similar to anadromous salmon than to non-anadromous salmon (Parsons 1973).  Rather than indicating that the Lake Ontario form was anadromous, however, this suggests that the conditions in the sea and Lake Ontario were more favourable for growth than those in relatively small lakes inhabited by non-anadromous salmon. 

Of some concern to the re-establishment of Atlantic salmon in Lake Ontario is a condition of thiamine deficiency that has been linked to a diet rich in alewife and smelt.  Both of these species contain high levels of thiaminase, an enzyme that catalyzes the destruction of thiamine (Ketola et al. 2000).  Results of several studies demonstrated that recruitment in non-anadromous populations in New York has been compromised by severe thiamine deficiency (Fisher et al. 1996, Ketola et al. 2000).  Thiamine deficiency can result in high mortality rates of eyed eggs (Ketola et al. 2000) and swim-up fry during yolk absorption (Fisher et al. 1996, Fynn-Aikins et al. 1998).  This condition has been called "Cayuga syndrome", "early mortality syndrome" (EMS) or "swim-up syndrome" (Fynn-Aikins et al. 1998).  Alewife and smelt colonized Lake Ontario in the late 1800s and early 1900s, respectively (Christie 1972).  These non-native species are the primary prey fish for salmon and trout in the lake today.  The dependence of Atlantic salmon on a diet of alewife and smelt lends support to the concern that thiamine deficiency may be affecting natural reproduction.  Research about early mortality syndrome is ongoing.


Interspecific interactions and survival 

Factors affecting the survival of Atlantic salmon change with each life stage.  Survival from the egg to alevin stage is dependent upon stream temperature at incubation and hatch, the amount of oxygen permeating the spawning gravel, the degree of siltation and the stability of the nest (i.e., whether it is re-excavated by another spawning female) (Mills 1989).

Survival from the fry to parr stage is primarily dependant upon optimal stream temperature which facilitates growth, and the nature and degree of competition with other salmonids for both food and territory (Gibson 1981, Mills 1989, Scott et al. 2003).  Juvenile rainbow trout are typically more aggressive than Atlantic salmon of the same age; however, this does not automatically translate to superior competitive ability (Hearn & Kynard 1986, Volpe et al. 2001).  While the two species demonstrate some degree of habitat overlap, and thus potentially engage in inter-specific competition, juvenile Atlantic salmon are more associated with positions close to the substrate (riffle areas) and rainbow trout with the water column (or pool habitats) (Hearn & Kynard 1986, Volpe et al. 2001).  Recent research conducted in Lake Ontario streams also suggests that Atlantic salmon and rainbow trout juveniles can coexist successfully in streams where the habitat is suitable for both species (Stanfield & Jones, 2003).  Coho salmon is another non-native salmonid species that through stocking has established a large, self-sustaining population in Lake Ontario (Scott et al. 2003).  The presence of chinook in the Lake Ontario system at very high densities has the potential to negatively affect Atlantic salmon behaviour and restoration efforts (Scott et al. 2003).  Yet another limiting factor for the survival of young-of-the-year fish is the threat of predation by larger Atlantic salmon, other resident fish species and piscivorous birds (Mills 1989).  The possibility for negative interactions between Atlantic salmon and other salmonine species such as rainbow trout (Onchorhynchus mykiss) poses a key concern to Atlantic salmon restoration (Stanfield and Jones 2003). 



There is a recognized association between ‘healthy’ Atlantic salmon populations and pristine or stable natural environments.  This species has demonstrated a low tolerance to environmental degradation and thus has been considered by many researchers as a potential indicator of environmental decline (Wilzbach et al. 1998).  It is important to note, however, that it is the changes wrought by disturbances that cause Atlantic salmon to suffer; once the rate of change has slowed; the salmon can adapt and potentially flourish in the new environmental steady state.

The life cycle of the Atlantic salmon is very complex (Figure 5), involving many different habitats and physio-chemical requirements.  Increases in stream temperature above the optimal range can compromise the incubation of eggs, the emergence of fry, the growth of juveniles, and the timing of smoltification.  Increased sediment loads and siltation can smother eggs in the nest by filling in the interstitial spaces in the gravel and can also restrict the ability of the visual feeding juveniles to capture prey items.  Natural disturbances can be equally as catastrophic as human-induced disturbance to Atlantic salmon populations.  A mid-winter ice breakup and subsequent flood in Catamaran Brook, N.B. scoured salmon nests, causing significant egg mortality (Cunjak et al. 1998).

Atlantic salmon have been raised in hatcheries since approximately 1866 when the federal Newcastle Hatchery was first established on Wilmot Creek (McCrimmon 1950).   The resulting fry and smolts have been stocked in many tributaries of Lake Ontario.  Traditionally Lake Ontario was stocked with non-native Atlantic salmon, but recently, researchers have been searching for remnant specimens of the original Lake Ontario stock.  One possibility had been that some fish of the Lake Ontario basin (NY side) were stocked into waters of Argentina; however, further investigation has suggested that the Argentinian fish originated from the Grand Lake stream hatchery identified in McCrimmon & Gotts (1979) (pers. comm., Gerry Smitka). Comparisons of DNA samples from Argentina, Maine (West Grand Lake, Sebago Lake), and Lake Ontario (wall mounts and vertebrae samples from the 15th century from an archeological dig along the north shore of Lake Ontario) will be conducted to confirm their relatedness.