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

Limiting Factors and Threats

The Atlantic salmon, a member of the original aquatic community of Lake Ontario, is no longer a naturally occurring or reproducing entity within the lake.  Many conditions contributed to the decline and ultimate disappearance of the salmon in Lake Ontario; however, most are attributable to the settlement of the basin and the alteration of habitat by timbering, agriculture, dams for mills, and the fishing industries (Dymond 1965).

In clearing the land for timber harvest and agriculture, destruction of the salmon’s natural environment occurred, since land clearance caused greater and more frequent fluctuation in water levels and periodically produced drought conditions in tributary streams in the summer months (Dunfield 1985).  Total land clearance also resulted in soil erosion and silting which degraded critical spawning habitat and is reported to have “suffocated” eggs and fry; hydrochemical changes which altered pH and shifted the balance and concentration of chemicals and nutrients within the stream; and the elevation of water temperature beyond the optimal range for Atlantic salmon (Dunfield 1985).

Atlantic salmon were extensively harvested by early settlers (~ 1792).  The species was popular due to the size of the fish, its abundance in all of the major tributaries to Lake Ontario, and the variability it provided to the settlers' diet (Dunfield 1985).  By 1840, salmon fishing emerged as a commercial venture in Lake Ontario; by 1866, the fisheries on both the American and Canadian sides of the lake had largely collapsed (Dunfield 1985).  Dymond (1965) suggested that while overfishing contributed to the demise of the Atlantic salmon, its influence was in addition to the habitat impacts and was not the sole cause.

Perhaps of greatest consequence to Atlantic salmon populations over their global range has been the construction of mill and driving dams for the timber trade, as well as dams for tanneries, carding mills and gristmills (Dymond 1965, Dunfield 1985).  These dams were typically built with no provision for fish passage so salmon were prevented from reaching their native spawning grounds.  The fish congregating at the base of the dams were vulnerable and more easily captured by fishermen. In many instances, this resulted in complete elimination of runs of Lake Ontario Atlantic salmon.

It is not possible to determine the date that the last native salmon was caught in Lake Ontario.  The last salmon generally believed to be from Lake Ontario, was a 7 lb (3.2 kg) fish reported by the Globe and Mail to have been caught off Scarborough Beach in April 1898 (Carcao 1986).

Several challenges to the re-establishment of the Atlantic salmon in Lake Ontario have been mentioned within this document and include: significant changes in the fish community, stream habitat quality, availability of a suitable genetic strain for stocking, possible effects of thiamine deficiency on reproduction and fitness of adults, and interspecific competition, especially in the streams.