Limnol. Oceanogr., 44(2), 1999, 371–381

It is possible to increase the sampling resolution and the available information on zooplankton populations by applying acoustic sampling techniques. Knowledge of an organism’s acoustic target strength (TS) is critical for translating acoustic data into meaningful biological information, such as numerical abundance. We have taken several approaches to evaluate the acoustic TS of Mysis relicta, a key benthic-pelagic invertebrate in the Laurentian Great Lakes. This included in situ TS measurements with a dual-beam 420-kHz system and comparison among netbased, optical plankton counter (OPC)-based, and acoustic-based estimates of numerical abundance of mysids. In addition, based on the in situ TS data, we modified a model previously applied primarily to marine zooplankton taxa. The resulting predicted TS was compared to other TS estimates. The estimated mean TSs from the in situ TS measurements and the OPC-acoustic comparison were within a narrow range (276 to 274.8 dB). Analysis of the net-acoustic data comparison resulted in an estimated average TS of 273.1 dB. We suggest that the net-acoustic discrepancy relative to other methods is due to net avoidance and low filtering efficiency. The model predicted similar mean TSs for the size distribution present when R (reflectivity coefficient) was changed to 0.0675. We hypothesized that the modification of the model parameter values was necessary because of the differences in the medium surrounding marine vs. freshwater zooplankton. The opossum shrimp, M. relicta, is a freshwater species in the mainly marine group of crustaceans belonging to the family Mysidaceae (Mauchline 1980). It is a benthic-pelagic invertebrate that inhabits offshore, hypolimnetic waters. Mysids avoid light, remaining in deep water during the day and moving up to the epilimnion at night (Beeton and Bowers 1982). The extent of vertical migration is primarily lim-