Figure 4: An echogram showing a deep scattering layer at more than 2000m, and the depth zones fished by the trawl.
The use of acoustics (recording of sound reflected from organisms) to record and quantify marine life has traditionally been associated with studies in waters overlaying continental shelves.
Recording conditions worsen with depth due to the limited range of scientific echosounders and by noise created by the research vessel (i.e. the signal to noise ratio prevents reliable recording to depths greater than 1000 m).
The ability to census aquatic organisms are even more difficult in deep water as many fish species here lack a swimbladder – the major acoustic reflector in most shelf species.
During the MAR-ECO cruise we used a SIMRAD EK-60 18 kHz scientific echosounder. When combined with the very low noise level of the RV G. O Sars, we were able to detect and quantify acoustic layers to depths between 2000 and 3000 m.
We were surprised by the geographical extent of layers recorded at about 1500 and 2300 m depth that were composed of fish without swimbladders (e.g. Bathylagus sp. and Maulisia sp.).
Scientists to be credited: Olav Rune Godoe, John Horne, Stein Kaartvedt, Caristiona Anderson, Ruben Patel