Echosounders are important tools for finding how much and what kind of fish are in the sea. Fishermen want to trawl on schools of fish of the right species and size. Marine scientists want to map which resources of fish and plankton are found in the sea. In both cases, echosounders provide useful information about what is in the water masses under the boat. This applies in particular to new types of broadband echosounders that can measure a continuous frequency spectrum. This type of broadband data gives opportunities for new methods that provide increased insight. This is a main focus for CRIMAC.
CRIMAC is an SFI center that aims to develop scientific methods for understanding and utilizing data from broadband echosounders. NORCE has relevant knowledge in acoustics, mathematical methods and data science and contributes with both theoretical method development and practical data processing. NORCE has also developed the software LSSS in collaboration with the Norwegian Institute of Marine Research, which is used to interpret acoustic data.
It is the swim bladder of fish that gives the greatest response when measuring with echosounders. For species without a swim bladder (such as sand eel and mackerel), the response is given by the body/tissue of the fish. Once the acoustic dispersion from the fish/swim bladder is understood, it is possible to say something about the characteristics of the fish based on acoustic measurements. The shape of a fish/swim bladder can, for example, be determined using ultrasound, and the corresponding scattering problem can then be solved numerically. Alternatively, a simpler model of fish/swim bladder can be defined which can be studied analytically. Both methods have advantages and disadvantages. NORCE contributes with the development of analytical methods where the fish's swim bladder is modelled as a prolate spheroid, i.e., a rotational ellipsoid which has been arrived at by rotating an ellipse around its long axis. The figures below show a sand eel, an ellipse and a prolate spheroid.
