Pre-catch sizing of herring and mackerel using broadband acoustics

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 algorithm is also applied to Atlantic mackerel (Scomber scombrus L.), with similar results. Horizontal broadband backscatter was measured from tethered fish at all rotation angles and used to develop and test three complimentary size estimation methods based upon target tracking, target strength, pulse stretching and spectral analysis of resolved targets. A 3-degree beam angle transducer affixed to the bottom of a ship's drop-keel, with mechanically controllable pointing angle, operating from 160 to 260 kHz was then used to collect data from individual fish in and around schools. Software was developed to implement these methods in real-time and applied to the at-sea dataset. The system shows promising results on adult herring and mackerel, but should be tried further on other size groups and fish species.