Dynamics and Exploitation of Lake Whitefish in Southern Georgian Bay

Instability of the population of Coregonus clupeaformis (Mitchill) in southern Georgian Bay from 1948 to 1964 was evidently partly due to intensive fishing.The population was discrete at least from 1956 to 1964 as determined by extensive tagging. From 1948 to 1964, estimated effective fishing effort varied 10-fold and annual yields almost 20-fold, with two cycles of abundance and scarcity. Strength of 1951–59 year-classes, defined as population size at age III, varied 40-fold. In spite of the marked instability, the population was in recent years more stable than any other whitefish population in Georgian Bay or Lake Huron.Almost all the commercial catch was made with gill nets of [Formula: see text] mesh. In 1957, fishing depended largely on age VII–IX fish; by 1964, on those of age V and VI. Apparent growth rate increased during this period: in 1957 age XII fish weighed 3.4 lb; in 1964 age VI weighed 3.7 lb. From 1959 to 1964, reproduction depended largely on males age IV+ and older and longer than 17 inches, and on females age V+ and older and longer than 18 inches.Natural mortality coefficients (exponential model) averaged 0.41 for fish of the 1948–58 year-classes beyond age III. Gillnetting was largely restricted to 2 months in late spring and two in fall. In each fishing season these gill nets took about 40% of the survivors of a year-class after it became fully vulnerable, at ages V, VI, and VII during the early 1960s.Combining growth and natural mortality rates in a biomass curve indicated that individual year-classes would provide highest yields when fished intensively at age IV, or slightly earlier than they were fished in the early 1960s. Yet the only hypothesis that might explain the continued existence of a fishable population was that relatively large parts of the habitat were either legally closed or unsuitable for fishing. Evidently the fishing was intense and increasing the effort would mean risk of a population collapse.