The Sea Lamprey in the Great Lakes

The sea lamprey was inadvertently introduced above Niagara Falls by the development of the Welland Canal between Lakes Ontario and Erie. A major population did not develop in Lake Erie but the species rapidly established itself as a highly significant predator in all three upper lakes. Its most obvious effect was the virtual extermination of the lake trout which had been the mainstay of the fishery. Efforts were made to limit sea lamprey reproduction by blocking the major spawning runs. These measures helped define the scope of the problem and generated considerable knowledge of the fluvial phase of the animalˈs life history but apparently exerted no significant restraint on population growth. Later control measures employed lamprey specific, selective toxicants to destroy larval populations in stream and estuarine habitats. Introduced first on Lake Superior to conserve the only significant lake trout stock then remaining, vigorous prosecution of these “treatments” resulted in a reduction, in 1961–62, of the Lake Superior adult sea lamprey population to a fifth or less of its maximum level. Since then the decline has continued though much less precipitously. A marked decrease in the proportion of males to females has accompanied the reduction in numbers. Similar changes in the sex ratio of spawning run Lake Michigan sea lamprey presumably reflect successful extension of control measures to that lake. Treatment of lamprey infected Lake Huron tributaries is not yet complete. Further reductions in lamprey numbers can reasonably be expected to result from improving treatment techniques but it is clear that total extermination is an unrealistic goal. It is not yet clear whether the present degree of control will permit restoration of self sustaining stocks of lake trout and other depleted prey populations.