Benni and Agust went to Norway recently to test the new Echo Sounder Technology. The purpose of the Echo Sounder monitoring test was to investigate if the Echo Sounder is a helpful tool to find the depth that bulk of the salmon is occupying in large sea cages.
In Norway, sea cages can be up to 160 meters in circumference and 40 – 50 meters deep.
The Echo Sounder used was the ES15 prototype. The transducer frequency is 192 kHz and its angle 32°. A wireless Ethernet network was created to connect the ES15 to a local PC. On the PC the Simrad ER60 Scientific Echo Sounder software was installed to display and record the test.
The transducer was strung on a rope that was hanging over the cage. It was at a depth of approximately 30 cm of and facing downwards. The Echo Sounder was therefore recording the reflections coming from the salmon below the transducer.
Facts about their observation and findings
The first test was performed at the Solheim cage-laboratory site of the Institute of Marine Research, Matre. The cage was 12 m x 12 m wide and 12.5 m deep and contained approximately 7.500 salmon.
The next observation was performed in Langholmen a Bremnes Seashore site just outside Hjelmeland in Rogaland.
The sea cages at the Langholmen site have a circumference of 160 meters and are 45 meters to the bottom of the cone. It contained 180.000 salmon and the biomass was 18 kg/m3.
Live temperature measurements were taken in the cage in 10 meter intervals. The temperature ranged from 16°c in the top of the cage and down to 11.7°c in the bottom during the observation period.
The conclusions from the observation tests was that in the larger cage with circumference of 160 meter and depth of 45 meters the salmon is able to swim more freely than in the smaller 12 x 12 meter and 12.5 meters deep cage. In the larger cage the salmon has more flexibility to find a place in the cage where it seeks optimal temperature, oxygen and/or some other factors that suits the salmon.
As displayed by the Echo Sounder in Langholmen the salmon shoaled at the bottom of the cage and up to 20 meters. At Langholmen the Vaki Biomass frame was at a depth of 6-7 meters. A more optimal depth for the frame would have been at around 30 meters or even deeper.
The Echo Sounder clearly displayed where the mass of the salmon population was and also where it wasn’t and in this case the population density was low in the upper 20 meters.