Biological Basis for Management of Lake Whitefish in the Michigan Waters of Northern Lake Michigan

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Abstract
Stocks of lake whitefish (Coregonus clupeaformis) have supported an intensive commercial fishery in the Michigan waters of Lake Michigan for over a century. However, certain biological indicators suggested that recent upsurges in the catch reflect overfishing, and fish managers should institute measures to assure a stabilized population and fishery. A biological basis for establishing quotas is described in this paper, using information from the 1968–73 commercial fisheries in statistical districts MM‐1 and MM‐3 and a modification of Rickerˈs dynamic pool model. Natural mortality rates computed for an unfished population of whitefish in the lower end of nearby Grand Traverse Bay were important components of the model. Quota possibilities were based on the premise that the annual harvest should be confined to weight gained each year by the harvestable portion of the population. Six computations of equilibrium yields were made. A comparison of actual harvests and adjusted yields revealed an average annual over‐harvest of 28% during the period 1968–72 in the two statistical districts. Total biomass for six age groups (I‐VI) in three Michigan statistical districts of northern Lake Michigan was computed to be 6,660 tonnes in 1972. Approximately 2,695 tonnes (60%) of the total biomass in MM‐1 and MM‐3 were susceptible to exploitation. Adjusted 1972 yields based on both biomass calculations and the modified dynamic pool model differed by only 0.3%. A change in the minimum total length limit (from 432 to 482 mm) to build up a depleted stock also was discussed. Increased spawning stock, more spawning opportunities, and greater egg deposition should result from this regulation change.

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