A new in-situ method to estimate fish target strength reveals high variability in broadband measurements

Abstract

Fish Target Strength (TS) is a requirement for estimating abundance from surveys of pelagic fish using echosounders. This paper describes a novel in-situ method to estimate TS as a function of fish body length (L), according to the standard equation TS = 20 log (L) +b20, where b20 is the species-specific factor to be estimated. We made measurements of TS with a broadband scientific echosounder, which is becoming the new tool of choice in fisheries acoustics due to its enhanced signal-to-noise ratio, and higher range resolution and the ability to measure the frequency response. A split-beam 38 kHz broadband transducer was pole-mounted on a small boat in the fjords of Nuuk, Greenland. With the boat stationary, individual Atlantic cod Gadus morhua were detected as echotraces, caught with a fishing line, measured in length and then released. Video footage, from a camera attached to the line, was inspected to verify a single individual was attracted to the lure and caught. Fish echotrace tracking techniques were applied to estimate the tilt of the fish from each acoustic sample and the measured TS was adjusted accordingly with the use of a Kirchhoff-Approximation scattering model. The b20 parameter was estimated by linear regression. This method combines the benefits of associating TS from single fish of known length, which is typical of ex-situ methods, with the in-situ advantages of measuring TS of an undisturbed fish in its natural environment. Results yielded a b20 of − 65.6 dB ( ± 0.83 dB C.I.), which is within the range of previously published values for Atlantic cod measured with narrowband systems. Large variability of TS was observed within fish tracks (average s.d. of 5.55 dB). This is the first description of TS for Atlantic cod with broadband equipment. The high variability could not be attributed to variation in fish length nor tilt angle. Other physiological and behavioural aspects were also discarded. It is possible that this may be an intrinsic property of broadband acoustics, which indicates a need for re-evaluating TS to length relationships as fisheries science moves towards the wider application of broadband acoustics.