Preliminary acoustic results from the New Zealand IPY-CAML survey of the Ross Sea region in February-March 2008

During February and March 2008 New Zealand carried out a major research voyage into the Ross Sea region in support of the International Polar Year Census of Antarctic Marine Life (IPY-CAML). The 50 day voyage on the research vessel Tangaroa involved an extensive survey of marine organisms from viruses to pelagic and demersal fish and cephalopods in depths from the surface down to 3500 m, and from the continental shelf and slope of the Ross Sea to unexplored seamounts and abyssal plains immediately to the north. Multifrequency acoustic data (12, 38, 70, and 120 kHz) were collected throughout the survey. Mark identification was achieved using targeted midwater trawls. Additional midwater and demersal trawls were carried out at randomly selected locations over the shelf as part of the core biodiversity survey. The main target species of the acoustic survey work was Antarctic silverfish (Pleuragramma antarcticum). Silverfish are a key link between plankton and the community of top predators in the shelf waters of the Ross Sea, but little is known of many ecological and biological aspects of this species. Silverfish were widely distributed over the Ross Sea shelf. Adult silverfish tended to form layers at 150–450 m depth and were sometimes present close to the bottom, where they were frequently caught in demersal trawls shallower than 500 m. A weak layer at about 80 m depth was found to be associated with juvenile silverfish of 40–80 mm standard length. The other major source of acoustic backscatter on the Ross Sea shelf was ice krill (Euphausia crystallorophias). Further north, over the slope and abyssal plain, acoustic marks were associated with myctophids (Electrona spp) and Antarctic krill (Euphausia superba). Acoustic backscatter from both silverfish and krill marks increased with increasing frequency (i.e., was highest at 120 kHz), which is characteristic of species without an air-filled swimbladder. Acoustic target strength (TS) estimates for silverfish at 38 kHz were estimated from anatomically detailed scattering models based on CT (computed tomography) scans of thawed specimens. The derived relationship between TS and fish length was unusual, having an extremely high slope for fish less than 11 cm. Preliminary estimates of acoustic biomass of silverfish in the Ross Sea based on this TS-length relationship were probably not credible, suggesting a very high biomass of juveniles (3 809 000 t) and much lower biomass of adults (118 000 t). Biomass estimates were also calculated for krill, with preliminary estimates (based on the krill TS of Greene et al. 1991) of 517 000 t (mainly E. crystallorophias) on the shelf, and 471 000 t (mainly E. superba) further north.