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Applying acoustic models to determine krill quantities

G.O Sars is equipped with six hull mounted echo sounders and one sonar which continuously emit sound waves. The returned echo signals are compared to acoustic models in order to identify krill and other species as well as to determine their size.

Picture above:
Rolf Korneliussen explains how the acoustic models are applied.

- This is the first time we have applied echo sounder data with different frequencies in order to determine the size of phytoplankton, says Rolf Korneliussen who is in charge of the acoustic measurements on the research cruise.
Knowing the variation in echo signal strength according to length is vital when it comes to determining the size of schools of krill detected by the echo sounders.

Data from the echo sounders not analyzed.

- By presuming that a given echo signal is of a krill six centimeters long we get a very different quantity estimation than if we presume the echo signal is from a krill three centimeters long, says Rolf.
The range in frequencies at which the various echo sounders operate give added insight. Large krill give the strongest echo signal at 70 kHz, small krill at 120 kHz. In the acoustic models the variation in echo signals are both compared and contrasted in order to determine quantity.

The same echo sounder data as above after being categorized.

The models are also validated in order to see how accurate they are. This is done by catching the some of the schools of krill that have been detected – and measured – by the echo sounders. The size of the caught krill is then measured manually.

- The catches tell us if the models give correct quantitative estimates. If not, we have to adjust the parameters for the models, says Rolf.
Several different models have been developed, one for krill and amphipods, another for ice fish and lantern fish, to mention a couple.