Depth-dependent target strength of anchovy (Engraulis japonicus) measured in situ

Abstract

Three sets of target strength (TS) data were collected in the southern part of the Yellow Sea using a calibrated, 38 kHz, Simrad EK500 split-beam echosounder. Midwater trawl sampling showed that >97% of the catch by number was anchovy (Engraulis japonicus), with total lengths ranging from 6 to 15 cm, and that the arithmetic-mean length and root-mean-square length were 10.6 and 10.8 cm, respectively. The mean TS of anchovy in the 10-45-m layer was estimated to be -50.9 dB, with a 95% confidence interval of (-51.0, -50.8) dB. The TS data showed, however, a clear depth-dependence that was very close to and not significantly different from what might be expected according to Boyle's law. The TS model was estimated to be TS = 20 log L - 71.6 for the conventional relationship between TS and length, but TS = 20 log L- (20/3) log (1+z/10) - 67.6 when the depth (z, m) effect was included according to Boyle's law. These results may have a significant influence on abundance estimates of anchovy derived from acoustic surveys, both in the Yellow Sea and in other parts of the world. © 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved.