Target strength measurements of suspended live ommastrephid squid, Todarodes pacificus, and its application in density estimations

Average target strength (TS) measurements of free-swimming Todarodes pacificus are required for calculating abundance estimations using the echo integration method. In this study, 31 live T. pacificus (18.0–28.4 cm dorsal mantle length; [ML]) were suspended with mantle tilt angles horizontally similar to free-swimming orientation, and their TS were measured using a 38 kHz splitbeam echo sounder. Normalized TS by squared ML of T. pacificus ML was calculated as −73.1 dB: TS=20 log ML −73.1 dB. The density of T. pacificus at their fishing ground off the northern Sanriku coast, Japan, was then estimated first using the echo integration method with the obtained normalized TS measurements, and then using commercial purse seine fishery CPUE (catch (tons) of squid per haul of purse seine) data. The density of T. pacificus aggregations near the continental shelf edge and upper continental slope was estimated as approximately 50–400 individuals/100 m2 using both the echo integration method and CPUE data, both density results were similar. It was considered feasible therefore, to assess T. pacificus biomass directly using the echo integration method.