Vital Statistics of the Lake Whitefish in Three Areas of Green Bay, Lake Michigan with Comparison to Lake Michigan East of Door County, Wisconsin

Abstract

Lake whitefish (Coregonus clumpeaformis) were sampled from commercial impoundment gear in 1976-1977 in Big Bay de Noc, Chambers Island, and Peshtigo Reef of Green Bay, Lake Michigan to calculate the vital statistics of the stocks. Results were compared among areas and with results from North-Moonlight Bays of Lake Michigan (Humphreys 1978) to determine if discrete stocks exist. The presence of a large 1972 year class at Peshtigo Reef and Chambers Island in spring were similar to the fall North-Moonlight Bays, and its absence at Big Bay de Noc indicated stocks of whitefish in the former areas were similar, and discrete from the latter. Total mortality rates estimated for Peshtigo Reef and Chambers Island in spring were similar to the fall North-Moonlight Bays rate indicating movement of fish from Peshtigo Reef and Chambers Island to North-Moonlight Bays during spawning. Rates of tag return for whitefish tagged in North-Moonlight Bays (1975-1977) indicated substantial movement of whitefish to the Peshtigo Reef and Chambers Island areas and migration back to North-Moonlight Bays in fall to spawn. Growth, determined by instantaneous rates and mean back-calculated lengths, was similar for whitefish from all areas. Growth of the large 1972 year class was less than that of other year classes. Age at recruitment ranged from 3.3 to 3.5 years in Green Bay but was 3.7 years in North-Moonlight Bays. Length-weight regression equations indicated differences among areas and seasons, but weights predicted from the regression equation were similar among areas. Total instantaneous mortality rate was higher in Big Bay de Noc (0.79) than to Peshtigo Reef (0.53) and Chambers Island (0.52) indicating a younger age structure in Big Bay de Noc. Two assumed rates of instantaneous natural mortality, 0.34 and 0.47, for Green Bay yielded estimates of instantaneous fishing mortality for Big Bay de Noc of 0.45 and 0.32 and for Peshtigo Reef-Chambers Island of 0.20 and 0.06. Mean number of spawnings, based on mortality estimates, were 1.84 and 2.30 for whitefish from Big Bay de Noc and Peshtigo Reef-Chambers Island respectively. Yield per 1000 kg of recruits calculated for whitefish in Big Bay de Noc from the Beverton-Holt yield and Ricker equilibrium yield models differed by 12%. Higher yield per recruit was calculated for North-Moonlight Bays than for Big Bay de Noc. The Ricker equilibrium yield model fit for the Big Bay de Noc stock, predicted yield in excess of production by 19% for a natural mortality rate of 0.34, but production was greater than yield by 11% for natural mortality of 0.47.