A SONAR device, comprising: an echo signal acquisition module configured to acquire echo signals from a ping, from a plurality of reception spaces (RS 1) adjacent to each other, the echo signals being of a target comprising a fish school; and a target detection module configured to calculate a range of the fish school by searching, in a search range associated with the fish school, echo connections in the echo signals of the fish school in the plurality of adjacent reception spaces (RS 1), wherein searching comprises determining whether or not a strong echo area exists in an adjacent reception space (RS 1) having an echo position with a same depth and a same distance as at least one echo position included in this strong echo area in a reference reception space (RS 1); wherein the search range associated with the fish school is adjustable; and a target position prediction module configured to calculate a predicted position of the fish school based on the echo signals of the fish school acquired from at least two previous pings, by calculating a displacement vector of the fish school based on a range of the fish school calculated at least for two previous pings, wherein the predicted position comprises a predicted range, which is obtained by moving the range of the fish school calculated in a previous ping, in accordance with the displacement vector; wherein the target detection module is configured to adjust the search range of the fish school based on the predicted position of the fish school; wherein: the echo signal acquisition module is configured to acquire from the ping echo signals of a second underwater target different from the fish school; the target detection module is further configured to adjust the search range of the fish school based on a position of the second underwater target; and the target detection module is further configured to not include in the search range of the fish school, a position of the second underwater target, when the position of the second underwater target is different from a predicted position of the fish school. an echo signal acquisition module configured to acquire echo signals from a ping, from a plurality of reception spaces (RS 1) adjacent to each other, the echo signals being of a target comprising a fish school; and a target detection module configured to calculate a range of the fish school by searching, in a search range associated with the fish school, echo connections in the echo signals of the fish school in the plurality of adjacent reception spaces (RS 1), wherein searching comprises determining whether or not a strong echo area exists in an adjacent reception space (RS 1) having an echo position with a same depth and a same distance as at least one echo position included in this strong echo area in a reference reception space (RS 1); wherein the search range associated with the fish school is adjustable; and a target position prediction module configured to calculate a predicted position of the fish school based on the echo signals of the fish school acquired from at least two previous pings, by calculating a displacement vector of the fish school based on a range of the fish school calculated at least for two previous pings, wherein the predicted position comprises a predicted range, which is obtained by moving the range of the fish school calculated in a previous ping, in accordance with the displacement vector; wherein the target detection module is configured to adjust the search range of the fish school based on the predicted position of the fish school; wherein: the echo signal acquisition module is configured to acquire from the ping echo signals of a second underwater target different from the fish school; the target detection module is further configured to adjust the search range of the fish school based on a position of the second underwater target; and the target detection module is further configured to not include in the search range of the fish school, a position of the second underwater target, when the position of the second underwater target is different from a predicted position of the fish school. the echo signal acquisition module is configured to acquire from the ping echo signals of a second underwater target different from the fish school; the target detection module is further configured to adjust the search range of the fish school based on a position of the second underwater target; and the target detection module is further configured to not include in the search range of the fish school, a position of the second underwater target, when the position of the second underwater target is different from a predicted position of the fish school.