Trapnet Mortality and Vital Statistics of Lake Whitefish in Green Bay and Northwestern Lake Michigan

A total of 28,744 lake whitefish were tagged at North and Moonlight bays (NMB), Whitefish Bay, and Peshtigo Reef, Wisconsin, and Big Bay de Noc (BBN) and the Bark and Cedar Rivers (BCR) area, Michigan, and 205,610 lake whitefish were measured in 1975 through 1985 to determine vital statistics and fit predictive models. Also, 31 trapnet lifts were observed and 10 holding net experiments were performed in Green Bay and Lake Michigan to assess short-term mortality of sublegal (less than 432 mm long) lake whitefish caught in trapnets. Two discrete stocks were associated with spawning areas at NMB and BBN, and fish from the NMB stock were migratory, whereas fish from the BBN stock were not. Data suggested that more than 75% of lake whitefish of sublegal size (less than 432 mm) survive a trap net capture and release, although storms occurring while fish are in the nets may reduce survival. Crowding in the lifter bag and long sorting times were primary factors reducing survival. The harvest from the NMB stock was spread over ages 3- 5 primarily, but the harvest from the BBN stock was predominantly age 3 and 4 fish. The 1972, 1975, 1978, and 1980 year-classes were the most abundant year-classes in the 1975-1985 harvest from the NMB stock. In the BBN stock, the 1976, 1978, and 1980 year-classes were more abundant than others in the harvest from 1979-1985. Fish from the BBN stock were longer at annulus formation than fish from the NMB stock up to age 3; after that fish from the NMB stock were longer. Fish from the BBN stock were estimated to be exploited at a higher rate than those from the NMB stock (51 and 41% for BBN and NMB, respectively) in 1976-1985, and estimated total mortality rate was lower for fish from the NMB stock (0.70) than for fish from the BBN stock (0.80). Fish of age-7 and older had lower mortality rates than fish of younger ages. Fishing accounted for 64% of total mortality in the BBN stock, and 59% in the NMB stock. Average annual abundance and recruitment estimates were 3 million fish of age 3.5 and older and 1.8 million recruits for the NMB stock, and 1.4 million fish of age 3.5 and older and 1 million recruits for the BBN stock. Quotas were similar to recent annual yields. Equilibrium yield per recruit for both the NMB and BBN stocks was less at the current minimum size limit (432 mm) than the initial weight of recruits, and as the simulated minimum size limit increased, yields decreased and spawning biomass increased.This study was supported by University of Wisconsin Sea Grant and the Wisconsin Cooperative Fishery Research Unit