Dynamics of an Exploited Population of Lake Whitefish, Coregonus clupeaformis, in Northwestern Lake Michigan
Hastreiter, James L.
University of Wisconsin-Stevens Point, College of Natural Resources
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A total of 12,378 lake whitefish (Coregonus clupeaformis) were tagged at North and Moonlight Bays (NMB) and Peshtigo Reef, Wisconsin, and commercial catches were sampled in 1975 through 1982 to determine vital statistics and fit predictive models for the NMB stock in Green Bay and Lake Michigan. Lake whitefish inhabiting the vicinities of Bark and Cedar Rivers, Chambers Island, Peshtigo Reef, Whitefish Bay and NMB in spring were major components of the NMB spawning stock in fall. Legal whitefish tagged in fall at NMB and sublegal fish tagged in spring at North Bay and Peshtigo Reef moved extensively in the study area. For sublegal whitefish, tag loss was 33% greater than for fish of legal size in the year after tagging. The estimated average annual exploitation rate was 27% in 1976- 1982; it varied with age ranging from 17 to 33% for ages 3 to 12. Ages 4-6 had a greater instantaneous total mortality rate (1.042) than did whitefish of age 3 (0.981) and ages 7-12 (0.420). The instantaneous fishing mortality rate for ages 4-6 (0.489) comprised 47% of total mortality and for ages 7-12 (0.245) it comprised 58% of their total mortality rate. Lake whitefish of ages 3-5 comprised 84% of the number harvested during the study. Age 4 made up almost half of the total sample, and about 25% were age 3 and about 17%, age 5. The fall harvest was comprised of 13% by number of ages 7 and older, the spring harvest, less than 6%. The 1972, 1975, and 1978 year-classes of the NMB stock were more abundant than other year-classes in the harvest from 1975-1982. Growth, expressed as weighted mean back-calculated length at age, was not significantly different among years from 1975-1982. Because lake whitefish in the NMB stock may be growing at nearly the maximal rate for the species, there may be little compensatory scope for increased growth if mortality rate increases. Lake whitefish in the NMB stock were heavier in fall than spring. The estimated population of the NMB stock of whitefish of age 3 and older averaged about 3.5 million fish in 1975- 1982, ranging from 2.1 million in 1980 to 6.7 million in 1975. Estimated numbers of recruits averaged about 1.9 million 3-year-olds from 1975-1982, ranging from 4.9 million fish in 1975 to 0.8 million in 1976. No relation between parent stock size and abundance of the resulting year-class at age 3 was found with the Ricker stock-recruitment curve. The Ricker equilibrium yield model predicted a yield of 867 kg per 1000 kg of age 3 recruits, which indicates the fishery harvesting the NMB stock is inefficient because less weight is being harvested than is entering the fishery. Ninety percent of the whitefish harvested from the stock were larger than the critical size (435 mm) in 1975-1982. Equilibrium yield simulations predicted that as the minimum size limit (432-mm) increased or fishing mortality decreased, yield would decrease and spawning stock biomass would increase conversely, as fishing mortality increased, yield would increase and biomass of spawning lake whitefish would decrease. A regulation that would reduce fishing mortality may not be justified under current conditions of relatively stable, high harvests for eight consecutive years because it would decrease yield, and I found no evidence that it would increase recruitment.