James Clark Ross travel (cruise JR16003)

Members of the NERC British Antarctic Survey, MESOPP partners, as well as other international partners have spent the last 40 days onboard the RRS James Clark Ross (Cruise JR16003) investigating the marine polar ecosystem in the Atlantic sector of the Southern Ocean. Setting off from the Falkland Islands, they travelled to South Georgia to undertake the 21st annual survey of the distribution of Antarctic krill within the Western Core Box [Fielding et al. 2014]. After this they undertook an 8 day transect across the Polar front, the dominant hydrographic barrier between Sub-Antarctic and Sub-Tropical waters, collecting information on the distribution of mesopelagic fish and macronekton (Southern Ocean prey). Acoustic data, to describe the distribution of fish, were collected using a hull-mounted multi-frequency echosounder (EK60 operating at 38, 70, 120 and 200 kHz) as well as using traditional net sampling methods (Rectangular Midwater Trawl 25m2, RMT25).

Bathylagus sp. and Krefftichtys anderssoni photographed for morphological analysis during JR16003. Image courtesy of Tracey Dornan (University of Bristol/BAS)

A number of different mesopelagic fish were caught, such as “Rhombic lanternfish” Krefftichthys anderssoni (a small fish with a swimbladder that generates a big acoustic signal, compared with the size of fish) and “deep-sea smelt” Bathylagus sp. (no gas-filled swimbladder) – see the photo. It is these organisms, and how they are aggregated, that form the patterns in the acoustic data. A greater understanding of what fish species are present, and how they scatter sound will allow us to better interpret the acoustic data that we can collect on larger scales. The acoustic data and this interpretation will feed into the MESOPP project.

Acoustic measurements from the cruise. Upper: 38 kHz mean volume acoustic backscatter data (Sv, dB re1m), collected whilst crossing the Polar Front. Lower: Sea surface temperature (°C) along the cruise track. The blue layer between 700 and 800 m, the “deep scattering layer”, intensifies within the warmer waters. Schools of fish can also be observed in the surface waters (~100 m depth) as cohesive structures. Sunset was 23:00 (GMT) and sunrise was 06:00 (GMT), but it is hard to distinguish an obvious diel vertical migration of the scattering layers in this data.