Alewives and rainbow smelt in Lake Huron: midwater and bottom aggregations and estimates of standing stocks

The continued availability of adequate amounts of forage fish, primarily alewives Alosa pseudoharengus and rainbow smelt Osmerus mordax, is critical to the success of ongoing programs aimed at rebuilding lake trout Salvelinus namaycush populations and maintaining other salmonid stocks in Lake Huron. These forage species are distributed at middepths as well as on or near the bottom. Acoustic methods were integrated with midwater and bottom trawling to characterize the population and estimate the biomass of the forage stocks. The average sizes of alewives and rainbow smelt caught at middepths were smaller than those caught in bottom trawls; however, most size ranges in the bottom trawl catches were also present in the midwater catches. Subadult and adult fish (both species) were rarely caught concurrently in midwater and when they were caught together the fish were invariably large subadults and small adults. Biomass estimates for the pelagic component were determined from trawl catches and echogram counts. The regression of echogram counts (X) on trawl catches (Y) was Y = -2.69 + 0.983X (r² = 0.766) at the fish densities investigated. The pelagic biomasses of alewives and rainbow smelt in United States waters of Lake Huron were estimated at 17,200 t in July 1974, 22,000 t in July 1975, and 19,000 t in August 1976. Biomass estimates of the stocks in midwater were usually larger in spring than in fall, probably due to seasonal differences in distribution rather than in abundance. Estimates for the demersal component of the combined alewife-rainbow smelt forage stock, calculated from stratified random sampling of the spring bottom trawl catches for 1973 through 1980 went from 35,000 t in 1973, to a high of 83,000 t in 1975, and to 72,000 t in 1980; the estimates in fall went from 31,000 t in 1973, to a high of 56,000 t in 1977, and to 43,000 t in 1980. Biomass estimates calculated from spring catch data were usually larger than those calculated from fall data. Estimates of the midwater stocks, coinciding with the spring and fall bottom trawl surveys, indicated that between 20 and 30% of the total biomass was in midwater.