Abyssopelagic fauna in the central North Pacific: comparison of acoustic detection and trawl and baited trap collections to 5800 m

Abstract

An integrated study using an acoustic array, opening-closing trawl and baited traps was conducted at an abyssal station in the central North Pacific (31°N, 159°W) to further characterize the near-bottom community. Two split-beam line arrays with beam patterns narrow in the vertical and omnidirectional in the horizontal were moored at 100 and 600 m above bottom (mab), sampling an insonified volume of 7855 m 3 each. In addition, a multiple opening-closing trawl (10 m 2 mouth opening) and baited traps were used to sample the fauna up to 1800 mab. Fourteen deployments of the acoustic arrays during two cruises detected 26 targets, with twice as many at 100 mab as at 600 mab (2.10 vs 0.97 targets h −1). Backscatter strengths for acoustic targets ranged from −57.2 to −26.8 dB. A total of 31 species, at least eight previously undescribed, were identified from trawl and trap collections within 1800 m of the sea floor. Of these new species, four of the decapod genus Hymenodora and one eel, Monognathus rosenblatti, were the most abundant animals collected by trawl. Other species commonly collected in baited traps included Acanthephyra quadrispinosa (Crustacea, Decapoda), Eurythenes gryllus (Crustacea, Amphipoda), and Coryphaenoides yaquinae (Osteichthyes, Macrouridae). Acoustic target abundances ranged from 0 to 1.6 animals per 10 5 m 3 per deployment, while values measured with the trawl ranged from 1.4 to 11.9 animals per 10 5 m 3. Biomass of the acoustic targets, estimated using a tentative identification based on size, and a series of regressions based on target strength and animal length and weight, ranged from 0 to 72.3 g wet weight per 10 5 m 3. Biomass estimates from the trawl samples ranged from 0.2 to 6.7 g wet weight per 10 5 m 3. The large number of new species collected during this study and the variability in sampling sparse populations using three different sampling techniques illustrate how little we know about the abyssopelagic community.