© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Application of in situ target-strength estimations in lakes: examples from rainbow-smelt surveys in Lakes Erie and Champlain
a Cornell Biological Field Station and Department of Natural Resources, Cornell University 900 Shackelton Point Road, Bridgeport, NY 13030, USA
b Lake Erie Unit, New York State Department of Environmental Conservation Dunkirk, NY 14048, USA
c Lake Erie Management Unit, Ontario Ministry of Natural Resources Box 429, Port Dover, Ontario, Canada N0A 1N0
d US Geological Survey, Vermont Cooperative Fish and Wildlife Research Unit, School of Natural Resources, University of Vermont Burlington, VT 05405, USA
*Correspondence to L. G. Rudstam; tel:+1 315 633 9243; fax: +1 315 633 2358. e-mail: rudstam{at}cornell.edu.
Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log10 L – 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (–60 to –58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.
Keywords: bubble production, fish density, hydroacoustics, smelt, target strength
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