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.
Lake Superior is highly oligotrophic and not very productive. It has not, however, been much affected by man's activities. Substantial quantities of woody allochthonous materials entered the lower reaches of tributaries and their estuaries from the pioneer lumbering industry. These undoubtedly depreciated the shallow-water benthic environment and may have adversely affected fish stocks in the immediate areas concerned, especially those of lake sturgeon (Acipenser fulvescens) and lake whitefish (Coregonus clupeaformis). At present, local areas of urban or industrial pollution can be identified but, in general, appear to have had little adverse effect on fish stocks, although the collapse of one walleye stock has been attributed to pollution from a paper mill. Every commercial species has, however, been severely depleted. Recent declines can be correlated with predation by sea lamprey (Petromyzon marinus) but, without exception, all species give evidence of extensive overfishing long before sea lamprey entered the lake. That this has not been more obvious is due to the stratification of catch statistics by arbitrary geographic areas (statistical districts) which are too large to relate meaningfully to either fish distributions or patterns of fishery operations. Because fish are generally distributed as local, often small, quasi-discrete stocks, they were exploited sequentially by the young, developing fishery. In this way stock after stock was depleted while conventional yield statistics gave an impression of relative stability. The lag before these statistics indicated the decline appears to be related to the degree of dispersal characteristic of each species. Thus the decline of the sturgeon was first evident, that of the lake trout (Salvelinus namaycush) last. Recovery has been slight but modern hatchery technology clearly provides a prospect for rejuvenating existing stocks or developing new ones. In either case, such synthetic populations will offer favorable opportunities for detailed investigation of the nature of fish distribution and its bearing on the interpretation of catch statistics.
The Lake Superior lake trout (Salvelinus namaycush) population is being rebuilt following its collapse in the early 1950s. Estimates are presented of the contributions to this recovery provided directly by the artificial recruitment of hatchery fish, a demonstrable amelioration in mortality rates and a resurgence, lately, of natural recruitment. Of the increased lake trout abundance, 55% on the average was owing to trebling the planting density, 40% to improved survival, and 5% to increasing recruitment of native lake trout. The precise contribution of the sea lamprey (Petromyzon marinus) control program could not be defined for lack of sufficient early data.Key words: lake trout, sea lamprey, rehabilitation, natural recruitment, hatchery stocking
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