Measuring fish length and assessing behaviour in a high-biodiversity reach of the Upper Yangtze River using an acoustic camera and echo sounder

Abstract

In the past decade improved acoustic hard- and software have enabled estimations of abundance and distribution patterns of aquatic organism, including non-intrusive monitoring of fish migrations and behaviour. In this study, a high frequency acoustic camera (DIDSON-LR, 1.2MHz, 0.7MHz) and a portable split-beam scientific echo sounder (Simrad EY60, 200kHz) collected acoustic data on 192 and 157 individuals within 24hr (19-20 April 2011) in the Mituo reach of the Yangtze River, China. Mean fish length estimated from the acoustic camera data was 18.7 +/- 5.6cm, with an average swimming speed of 0.19 +/- 0.13ms(-1). The mean fish target strength (TS) produced by the echo sounder was -43.8 +/- 4.4dB, which corresponded to 5.7-119.9cm fish length when converted by three different TS-length equations. Average swimming speed was 0.11 +/- 0.06ms(-1) from the echo sounder. Compared with the actual fish catch by the three layers of drift gill net in the survey area, the target length indicated by DIDSON was more accurate than the EY60 results, which were highly affected by the choice of TS length equations. It was determined that the two devices used synchronously could estimate fish length effectively to investigate their behaviour and distribution.