Multifrequency echo sounder operation
In cooperation with the Institute of Marine Research in Norway, we have developed an echo sounder system capable of discriminating between different fish species.
Recent research has shown that simultaneous use of several discrete echo sounder frequencies (multifrequency) not only improves fish stock estimates, but can also be used to identify species. This is because each specie has a unique acoustic frequency response. This new and growing understanding greatly improves the value of hydroacoustics to obtain information about marine resources.
Scientists at the Institute of Marine Research, Norway, have shown that different species of zooplankton and fish can be identified based on multifrequency acoustics.
In the future, databases of 'acoustic signatures' for the different commercially interesting fish species will be established and we are already developing echo sounders that can take this information into practical use.
- Single frequency echo sounders have traditionally been used to locate fish resources and to determine their size, both at population and individual level. With the high exploitation rate on limited fish resources seen internationally, selective fishing has become a major topic in fisheries management. Our goal is to provide echo sounders for the international fishing fleet being capable of both species identification and accurate size estimation. Sustainable exploitation of our fisheries resources is dependent on this information and any instrument being capable of providing such information will be a success. Our development of fishery research systems can help the industry as a whole, says Frank Reier Knudsen, fishery research scientist at Kongsberg Maritime.
The echogram below shows schools of mackerel. These are observed using frequencies 18, 38, 70, 120, 200 kHz on a Simrad EK60 scientific echo sounder.
The schools of mackerel can be seen in the left part of the echogram. It is clear that the echo from the schools becomes stronger with increase in frequency. This response is unique to mackerel and can be used to discriminate mackerel from other fish species.
The echograms on this page show a recording of a layer of krill using 18, 38, 70, 120 and 120 kHz. As opposed to the mackerel, the strongest echo from krill is obtained at 70 kHz. The frequency responses of mackerel and krill are clearly different and can be used to identify and discriminate these species.