Population Dynamics and Stock Identification of Rainbow Smelt in Green Bay and the Adjacent Waters of Lake Michigan

Abstract

Length, weight, and sex were recorded and scales were collected from rainbow smelt Osmerus mordax at 6 spawning locations: 2 in west central Lake Michigan, 2 in central Green Bay, and 2 in northern Green Bay; from the commercial trawl fisheries in west central Lake Michigan and central Green Bay; and from the commercial pound net fishery in northern Green Bay in 1990-1992. Vital statistics were estimated for smelt from the various locations and fisheries. Significant differences in age and length frequency distributions occurred among locations and years at several spawning sites. Differences among years at a location precluded a conclusion that these differences were attributable to the existence of separate stocks. Mean back-calculated lengths for all spawning samples combined were 83, 125, and 153 mm for males and 83, 126, and 156 mm for females at annuli I, II, and III, respectively. Growth of smelt from Turtle Creek, a central Green Bay spawning site, was significantly faster than growth of smelt from some other spawning sites and from those collected off-shore in trawls in central Green Bay. Smelt from Turtle Creek may represent a separate stock. Slopes of length/weight relations differed among spawning locations for male and female smelt but, these differences were inconsistent with differences in growth among sites. Length/weight relations of smelt collected from trawls differed between west central Lake Michigan and central Green Bay, but the differences were not the same for males and females, indicating that the differences were not due to separate stocks. Survival was lower for males than females. Mean annual survival rates (ages 3-4) for all spawning samples combined were 0.04 for males and 0.15 for females. Estimated instantaneous commercial fishing mortality of smelt was higher in northern Green Bay (0.55) than in central Green Bay (0.08) and central Lake Michigan (0.16). Estimated instantaneous natural mortality was higher in west central Lake Michigan (1.93) than in central Green Bay (1.57) and northern Green Bay (1.37). Higher natural mortality in Lake Michigan than in central and northern Green Bay may have been caused by predation by stocked salmonids. The estimated mean population size of smelt of ages 2-4 in commercially fished waters of west central Lake Michigan in 1992 and in central Green Bay in 1990 was 290,970,707 and 186,540,556, respectively, with the corresponding biomass of 4,158,421 and 2,665,863 kg. Estimated mean population size and biomass of smelt of ages 2-5 in commercially fished waters of northern Green Bay (Grids 306, 504, 505, and 604) were 108,153,609 and 1,524,217 kg, respectively. Estimates of equilibrium yield for smelt in the commercial trawl fisheries in west central Lake Michigan and central Green Bay, and in the commercial pound net fishery in northern Green Bay indicated that each fishery was inefficient, in that the estimated losses to natural mortality exceeded the estimated yields. Simulations with different rates of fishing mortality indicated that all three of the commercial fisheries were not reducing the stock(s) of smelt that they exploit.