B. W. Nagy and K. J. Hartman, West Virginia University Division of Forestry, Wildlife and Fisheries, PO Box 6125, Morgantown, WV 26505

Abstract. The use of hydroacoustics in fisheries research, while not entirely new, is relatively undocumented for most fish species and most waterways. It was therefore necessary to determine species-specific target strengths (TS) for both striped bass Morone saxatilis and white perch M. americana while performing abundance estimates on the Hudson River estuary. The TS measurements were taken through the use of a fish cage constructed of plastic mesh and aluminum supports, and a Simrad EY500 with a ES120 split beam transducer. A number of individuals and sizes for each species were caught using both mid-water trawls and hook and line. These fish were then placed, one at a time, in the enclosure and it was lowered to an appropriate depth to capture the entire fish in the acoustic beam. After each fish was held in the beam for several seconds and data for that fish was logged, the enclosure was retrieved and the process was repeated. On return to the lab, the data was analyzed using custom made programs written in the IDL programming format. Analysis showed, as expected, that the larger fish produced a larger TS, while smaller fish produced a smaller TS. The main portion of a fish responsible for the TS is the swim bladder, therefore the larger the fish the larger the swim bladder and the larger the TS.