An echo signal processing device to acquire a status of movements of fish comprising a detecting unit, a correspondence unit and an association unit. The detecting unit detects echoes of fish from echo signals generated by a sonar sensor (3). The correspondence relation identifying unit (17) identifies a correspondence relation between a first fish echo detected at a first time and a second fish echo detected at a second time. The correspondence preferably being determined by using the ship location to determine the position of a centre of the fish generating the signal and comparing the centres in the two signals. An association information generating unit (15) generates association information associating the first and the second fish echoes for which the correspondence relation identified (17) with each other. The association information preferably forming a vector indicating the direction of movement of the detected fish.
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