Some problems and solutions for the measurement of fish target strength: A study case with Atlantic redfish (Sebastes spp.)

Abstract

Potential biases in the measurement of redfish target strength (TS) were examined by comparing ex situ and in situ approaches. Ex situ experiments were conducted on individuals after developing a method to maintain live fish in good condition. The main problem with the manipulation of these species was the inflation and distortion of the swimbladder. To minimize this bias, fish were kept in sea cages after acclimation at depth and TS was measured in a camera-monitored apparatus set in proximity to the capture site. A series of in situ acoustic-trawl experiments was conducted on several aggregations of redfish in Newfoundland waters. TS were collected using a hull-mounted EK500 split-beam transducer and a deep-tow dual-beam system. The dual-beam transducer was calibrated at different depths to test if change in pressure and/or temperature affected its sensitivity. This system was used to measure aggregations of fish under different transducer depth. The data indicated that biases in in situ TS estimates increased with the range of observation, the density of fish, and the presence of multiple targets formed by the clustering of smaller organisms. Depending on the nature of the bias, TS can be over- or underestimated by as much as 6 dB.