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Listening for marine life in the deep ocean

A recording of vertically migrating organisms from the seabed-mounted echsounder.
When observing life beneath the surface of the sea, echosounders are the “eyes” of the marine scientists. Echosounders utilize sound in much the same way as our eyes or cameras use light. The reason for preferring sound to light is that sound travels well in water and many animals reflect sound. This reflected sound, actually the echo, is recorded by the echosounders. Post-processing of these signals facilitates studies of distribution and abundance of the sound-reflecting animals ranging from small zooplankton to marine mammals.

The group of scientists assembled on the jetty at Espegrend on a beautiful winter day. From left: John Horne (USA), Tone Falkenhaug, Anders Opdahl (MSc student, Norway), Kirsty Anderson (UK), Stein Kaartvedt (Norway), Olav Rune Godø (Norway), Henrik Søiland, Kristina Arianson (MSc Student, Norway), Leif Nøttestad (Norway), Odd Aksel Bergstad. Not present: Thomas de Lange Wenneck, Tracey Sutton.
Assembled for five days in the University of Bergen field station at Espegrend, MAR-ECO scientists made significant progress on post-cruise analyses of the data from echosounders. The post-processing involves combining data from the echosounders with observations made by samplers, e.g. plankton nets and trawls, or optical instruments. Therefore, the group consisted of specialists in hydroacoustics, and biologists studying occurrence and abundance of plankton, fish and mammals using sampling gears and visual sightings.

Echosounders mounted on the hull of the vessels and on observation platforms sitting on the seabed were used by MAR-ECO, and an enormous dataset was obtained. From the hull-mounted sounders, data were collected from the entire cruise track. This reveals how the abundance of sound-scattering animals varies on a large geographical scale and by depth zones. Typically, there are several layers of organisms each with discrete depth-distributions. One of the more exciting findings from the 2004 G.O.Sars expedition was that full ocean depth observations were attained by the new echosounders. The data have still to be synthesized and analysed fully, and one of the objectives of the Espegrend meeting was to make progress towards a full presentation of the results.

An exceptional day at Espegrend.
The seabed-mounted sounders listen upwards, i.e. from the bottom towards the surface. These are battery-powered units and were designed in partnership with one of the MAR-ECO commercial partners, Kongsberg SIMRAD. These “Bergen Acoustic Landers” were deployed successfully on three sites on the mid-Atlantic Ridge in 2004. One of the landers stayed out for almost an entire year and provided an unbroken series of data from all seasons. A wealth of information resulted from this instrument, e.g. undisturbed recordings of vertical migration by fish and plankton, occurrence of diving whales, single-fish behaviour patterns, and surface-occurrence of fish schools. Many of these observations could not have been made by vessels, either for technical reasons of because of time constraints.

The Espegrend meeting stimulated further work of scientists and students and facilitated progress towards a number of scientific reports containing exciting news on the patterns of distribution and behaviour of marine life on the mdi-Atlantic Ridge.

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