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

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