Ex situ target-strength measurements of Japanese anchovy (Engraulis japonicus) in the coastal Northwest Pacific

The Japanese anchovy (Engraulis japonicus) is an important species in regard to the fisheries and ecology of the coastal Northwest Pacific. Measurements of ex situ target strength (TS; dB re 1 m(2)) were made on live anchovy using 38, 120, and 200 kHz split-beam echosounders. The fish were tethered using small hooks attached to their mouths. During the acoustic measurements, an underwater video camera was used to continuously monitor fish behaviour and tilt-angle (theta). Data for 35 individual anchovy ranging from immature to adult sizes (total lengths L-T = 4.8-12.2 cm) were analysed. Least-squares regression fits of TS vs. log(L-T) were: TS38 kHz = 20 log(L-T) - 65.8 (r(2) = 0.82), TS120 kHz = 20 log(L-T) - 68.4 (r(2) = 0.84), and TS200 kHz = 20 log(L-T) - 69.1 (r(2) = 0.71). The LT vs. wet weight (W; g) relationship for these fish was W = 0.0036 L-T(3.204). The mean theta for anchovy swimming freely in a large seawater tank was 9.1 degrees (s.d. = 13.1 degrees). These ex situ measurements of TS, L-T, W, and theta can be applied to improve acoustic estimates of Japanese anchovy biomass.