Sonars and echosounders are widely used for remote sensing of life in the marine environment. There is an ongoing need to make the acoustic identification of marine species more correct and objective and thereby reduce the uncertainty of acoustic abundance estimates.
The price of caught fish can depend strongly on their length or weight. This creates a strong motivation and need for pre-catch size estimation, particularly in purse seine fisheries. While it is customary to release unwanted schools, this can cause high mortality in the fish. We present initial measurements and at-sea trials of an acoustic system and real-time processing algorithm that estimates the length of Atlantic herring (Clupea harengus L.) to within +/- 3 cm.
The development of methods for the acoustic identification of fish is a long-term objective aimed at reducing uncertainty in acoustic-survey estimates. The relative frequency response r(f) measured simultaneously at several frequencies is one of the main acoustic features that characterize the targets, but the relationship between nearest neighbours, school morphology, and environmental and geographical data are also important characteristics in this context.
Combining multibeam-sonar and multifrequency-echosounder data: examples of the analysis and imaging of large euphausiid schools
The first high-resolution, quantitative, multibeam sonar (Simrad MS70) ever developed was mounted in a keel of RV “G. O. Sars” with port-orientated beams. Each ping samples a volume of 60° horizontally × 45° vertically with 500 beams, which is often enough to insonify a complete school of fish or zooplankton.