First in situ estimates of acoustic target strength of Antarctic toothfish (Dissostichus mawsoni)

Antarctic toothfish (Dissostichus mawsoni) is a large nototheniid endemic in Antarctic waters. They are a top predator, and an important commercially fished species with a circumpolar distribution mostly south of the Antarctic convergence. Fisheries acoustics is a potential tool to estimate toothfish abundance and distribution, but previous studies have been limited by lack of information on the acoustic target strength (TS). In this paper we present the first in situ estimates of TS for Antarctic toothfish. Data were collected by deploying acoustic equipment through the sea ice in conjunction with vertical line fishing and baited underwater video (BUV) observations. Estimated mean TS from 250 tracked single targets detected in situ in Terra Nova Bay was -37.8 dB re 1 m(2) (95% confidence interval -38.2 to -37.5 dB). Estimates of toothfish length from BUV images of 42 individuals ranged from 92 to 201 cm total length (TL) with mean length 134 cm. Estimates from 15 Antarctic toothfish (104-153 cm TL, average 131 cm) captured using vertical lines in McMurdo Sound gave a lower mean TS of -40.2 dB (range of individual fish means - 38.3 to -43.7 dB). Although estimates from hooked fish have a number of potential biases, due to unnatural orientation and acoustic interference from fishing gear and adjacent hooked fish, these results supported the conclusion from our in situ estimates that toothfish have higher TS than previously thought. In situ acoustic observations showed most toothfish within 100 m of the seabed.