For the last two months (November 19th 2016 – January 19th 2017), Roland Proud (@rolandproud, Pelagic Ecology Research Group, University of St Andrews) has been on board the Akademik Tryoshnikov, recording echosounder observations of Deep Scattering Layers (DSLs) in the Southern Ocean as part of the Antarctic Circumpolar Expedition (ACE).

Southern Ocean mesopelagic biogeography (adapted from [Proud et al., 2017]). Mesopelagic classes (C1-C10) have characteristic Deep Scattering Layer depth and density (echo intensity) values. Intended Antarctic Circumpolar Expedition cruise track overlaid.

The mesopelagic (200 – 1,000 m) community comprise primarily of layers of fish (lantern fish, myctophids) and zooplankton. Acoustic observations of these layers using echosounders, which produce sound waves and record water-column backscatter, are known as Deep Scattering Layers (DSL). Deep Scattering Layers are ubiquitous features of the world ocean, but data in the Southern Ocean are sparse. Our previous work has revealed pronounced differences in Deep Scattering Layer depth and biomass across frontal zones and suggested the possibility of a global Deep Scattering Layer biogeography, proposed in [Proud et al., 2017] as shown above.

Example echogram taken from Proud et al. [2017]. Deep Scattering Layers (DSLs) are labelled. Increasing values of Mean Volume Backscattering Strength (MVBS) generally indicate an increase in biomass. Data from 38 kHz echosounder observations collected by the fishing vessel Will Watch (www.imos.org.au) whilst underway in the South West Indian Ocean.

The daily vertical migrations of animals in Deep Scattering Layers, from deep water (200 – 1000 m) in daylight to shallower water (< 200 m) to feed at night make an important contribution to the ‘biological pump’ that moves carbon from the surface to the ocean interior [Brierley, 2014]. Fish in Deep Scattering Layers are hunted by diving predators such as elephant seals and king penguins and are also potentially of commercial interest. Data on Southern Ocean Deep Scattering Layers are needed to improve ecosystem-based management and to improve our understanding of biogeochemical cycling and drivers of predator foraging behaviour.
The active acoustics project on Antarctic Circumpolar Expedition (ACE), “Circum-Antarctic distribution of acoustic deep scattering layers, and associated foraging behaviour of deep-diving predators”, is led by Professor Andrew Brierley (Pelagic Ecology Research Group, University of St Andrews) and will continue for the next two months as the ship makes its way around the Antarctic continent (follow ACE here: @ACE_expedition).

Roland Proud’s postdoctoral position is currently part-funded by MESOPP, and the University of St Andrews is partner of the project.