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Studying the krill without disturbing it

Acoustic measurements of krill are usually done by using echo sounders mounted on moving ships. Mounting echo sounders on platforms which are submerged and subsequently left alone for a day or two, however, yields a better understanding of the natural behavior of krill and other creatures found in the Antarctic waters.

Above: Atle Totland and Terje Torkelsen installing equipment.

The fish and krill that the researchers want to study might be disturbed by noise even from the most quiet research vessel. Furthermore, a moving ship is not particularly suited to give a good longer term understanding of what happens in the water the ship passes over. It provides at best a snapshot of each place, but in return it has the ability to survey large areas.

It is, however, interesting to study the activity in one place over a longer period of time with as little disturbance as possible. This can give a better understanding of the natural behavior of krill and fish, which for instance tend to act differently at night than they do during the day. This is also difficult to measure from a moving ship.

The Institute of Marine Research (IMR) has developed two platforms with echo sounders mounted in order to measure longer term natural behavior within defined areas. One platform is placed close to the bottom of the ocean looking upwards and the other one is placed closed to the surface looking downwards. Both types are used on this research cruise with G.O. Sars.

The red colour at the bottom of this picture is the surface. Blue areas are krill. This is taken from the echo sounder placed low looking upwards.

The acoustic release has been activated and the small platform has reached the surface.

- The platform which is looking upwards can study krill all the way up to surface level which is not possible from this vessel. The keel is 10 meters below surface and the echo sounders need another five meters or so before producing discernible echoes, Terje Torkelsen and Atle Totland explain.

For the first two weeks the two engineers were busy constructing the two platforms which were delivered in parts on the dock in Montevideo, Uruguay. They just finished construction in time for calibrations. Now the platforms have been deployed in several different locations in the waters around South Georgia. To retrieve the platforms an acoustic signal is used which activates the release mechanisms that detach the platform from their moorings after which they float to the surface. During transport to the next place of deployment, data is downloaded to computers on the ship.
- These platforms can do measurements without outside disturbance and thereby be able to record even small echoes, which is an advantage because the echoes from krills are not among the strongest, Atle and Terje point out.

IMR has now developed standardized module based platforms which can take payloads of 50 or 100 kilos depending on their use and which data logging devices is used. Current meters, hydrophones, stereo cameras, and CTDs can also be mounted on the platforms in addition to echo sounders.

The almost uninterrupted blue line above is probably a fish that the echo sounder was able to track for two hours.

- Fra disse plattformene kan målingene gjøres uten noe ytre påvirkning, uten støy. Og siden det ikke er noen ytre påvirkninger som forstyrrer, kan ekkoloddet oppfatte svake ekko. Dette er gjerne en fordel når krillen skal studeres, for den avgir ikke alltid et kraftig ekko, påpeker Terje og Atle.

Denne typen plattform ble også brukt under kartleggingen av Den midtatlantiske rygg for noen år siden (MAR-ECO-prosjektet). Nå har Atle og Terje standardisert plattformtypene og laget til et modulbasert system som inkluderer oppdrift, batterier, elektronikk og datalagring. Den minste plattformen kan ta med seg utstyr på opp til 50 kilo, den store 100 kilo. Valg av størrelse som brukes er blant annet avhenging av hvor mye utstyr man ønsker å benytte. I tillegg til ulike typer ekkolodd, kan også strømmåler, hydrofoner, stereokamera og salt-, temperatur- og dybdemåler med mer monteres på plattformene.