Changes in the Population of Lake Whitefish (Coregonus clupeaformis) in Lake Winnipeg from 1944 to 1969

Abstract

Whitefish catches have fluctuated since 1952, and except in 1964 each successive peak has decreased in magnitude. Recently the overall trend has been a diminishing fishery with increasing effort.There has been a drastic change in age composition with time. Age-groups 5, 6, and 7 accounted for 81% of the catch from 1944 to 1948, age-groups 4, 5, and 6 for 89% from 1949 to 1955, age-groups 4 and 5 for 86% from 1959 to 1968, and age-groups 3 and 4 for 88% in 1969. The fishery is now dependent upon two age-groups instead of three. Natural failure of one or more important year-classes would result in a serious decline in catch, and partial or complete collapse of this fishery.Annual survival rate decreased from 34%, 1944–48 to 12%, 1959–69. Instantaneous natural mortality rate was estimated to be 0.72, and may be overestimated. The 1938–39, 1945–47, 1949, and 1957 year-classes were strong, while the 1940–44, and 1956, 1958, and 1961 year-classes were weak. The 1959 and 1960 year-classes appeared to be somewhat above average. Examination of data on total dissolved solids and temperature indicated that ecological changes were not responsible for the decline of whitefish in Lake Winnipeg.We judge that the optimum rate of fishing has been exceeded, and that the fishery itself is largely responsible for the depleted condition of the whitefish stocks in Lake Winnipeg. Fishing effort must be reduced to the point where the fishery is again dependent upon three year-classes. This should result in higher catches, and provide less fluctuation in annual catches.