Exotic Species Endanger Native Fish

Non-native fish are already implicated in the extinctions of five fish species in Canada and they threaten the continued existence of many others.

Invasive species jeopardize 26 of the freshwater fish species, sub-species and isolated populations considered by the Committee on the Status of Species at Risk in Canada (COSEWIC) in 2003 as endangered or threatened.

Usually several factors in unison contribute to the declining numbers of fish at risk. Loss and degradation of wetlands, lakes and rivers are certainly the most widespread problems. Invasive species pose the second most prevalent peril.

Nation-wide, natural fish populations are being harmed by fifteen species of invasive fish, particularly pumpkinseed, sea lamprey, alewife and black spotted goby. Most of these fish were introduced into Canadian waters without sanction in attempts to improve sport fishing. Besides exotic fish, the aquatic plants Eurasian watermilfoil and reed-canary grass are also devastating for some local species.

Introduced animals and plants have made the greatest dent in native fish biodiversity within watersheds of the Great Lakes and upper St. Lawrence River, and on Pacific islands off the coast of British Columbia.

One widespread menace, rainbow smelt, got its start in 1912 when it was released into Crystal Lake to help feed trout. Smelts rapidly moved into all the Great Lakes and hundreds of smaller lakes in central Canada, aided by anglers using the fish as bait. They have already propelled blue pike to extinction and now threaten four other native species.

Brown bullhead, a type of catfish, is one of the worst offenders, affecting nine native species. Within years, bullheads gobbling up enough eggs to wipe out two stickleback species unique to Hadley Lake on Lasqueti Island in BC. The ongoing spread of brown bullheads seems to make extinction inevitable for four more stickleback species on nearby Texada Island, and two in Enos Lake on Vancouver Island.

In addition to the sticklebacks, native fish which are particularly struggling against the consequences of introductions are pugnose shiner, Cultus pygmy sculpin, Lake Utopia dwarf smelt, and deepwater sculpin in the Great Lakes. Invasive species have also furthered declines of the endangered Salish sucker, northern madtom, Atlantic whitefish, Cultus sockeye salmon, and Atlantic salmon of the inner Bay of Fundy. Some of these fish are unsuccessfully coping with more than one exotic species.


Alan J. Dextrase and Nicholas E. Mandrak. 2006. Impacts of Alien Invasive Species on Freshwater Fauna at Risk in Canada. Biological Invasions. 8(1): 13-24.

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