Comparison of target strength distributions and fish densities obtained with split and single beam echo sounders

Abstract

We compared results from analysis of split-beam and single-beam acoustic surveys using two data sets, one from the Baltic Sea (both systems at 70 kHz) and one from Lake Erie (70 kHz single beam, 70 and 120 kHz split beam). First, we show that there is a bias towards smaller targets associated with implementation of a commonly used algorithm for deconvolving target strength distributions from single beam data (modified Craig–Forbes). This bias is 0.8 dB for a circular transducer. Differences in water column fish density estimates were not large although the single beam analysis consistently resulted in lower fish densities (85–95% of split beam densities). Target strength distributions obtained by split and single beam methods were very similar for the Baltic Sea data resulting in almost identical overall average TS. This was also true for epilimnetic targets strength distributions in Lake Erie. However, hypolimnetic target strength distributions still yielded smaller target strengths with single than with the two split beam units. The split beam analysis yielded target strength distributions with a wider dynamic range and more detail than the single beam analysis. Nevertheless, the traditional single beam analysis gave results for larger targets (over −56 dB) that were comparable to the information gained from the split beam units.