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

Abstract

Abstract Zhao, X., Wang, Y., and Dai, F. 2008. Depth-dependent target strength of anchovy (Engraulis japonicus) measured in situ. – ICES Journal of Marine Science, 65: 882–888. 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.