To formulate appropriate management strategies in Lakes Michigan and Huron, state, tribal and provincial managers must have accurate and timely information about the prey fish populations and the interactions between predator and prey fishes. As part of a project with applications to all the lakes, hydroacoustic technology as a method for assessing and identifying pelagic (mid-lake, sometimes deep water dwellers) fish stocks is being evaluated. Currently, scientists must rely on mid-water trawling to derive species composition. Due to limitations of bottom bathymetry and gear biases, bottom trawls may not be the best tool to provide estimates of absolute abundance. Acoustic assessment holds the most promising method to accurately estimate abundance of the major prey fish species.

The objective of this research is to determine if broadband sonar can acoustically distinguish the major pelagic planktivores out in the lakes such as alewife, rainbow smelt, sticklebacks and bloater. It will determine if this method of fish stock assessment is practical and accurate, as well as if it is extendable to all the Great Lakes.

Results so far include Fleischer et al. 1997 (In situ relations of target strength to fish size for Great Lakes pelagic planktivores) in which concurrent hydroacoustic surveys and midwater trawls were conducted. Target strength proved to be a reliable predictor of fish biomass. Changes in mean target strength explained 73% of the variation in mean weight for schools of rainbow smelt, alewife, and bloaters. Bloaters tended to be less acoustically reflective than other species and a different linear regression was used for schools primarily containing bloaters.

Another manuscript by Fabrizio et al. 1997 compares fish densities and species richness from simultaneous collections with acoustic-midwater trawling and bottom trawling. In shallow water (18 m), bottom trawls detected more species but fewer fish than acoustic-midwater trawling. Biomass and density of bloaters was significantly higher when based on bottom trawl samples. Significantly fewer alewifes were estimated from bottom trawling at 55 m and no alewifes were taken by bottom trawling in deeper waters. The differences detected between gears resulted from alewife, bloater, and rainbow smelt vertical distributions, which varied with lake depth and time of day. Because Lake Michigan fishes are both demersal and pelagic, a single sampling method cannot be used to completely describe characteristics of the fish community.