Underwater detection apparatus for detecting a target object by acquiring underwater echo signals at fixed timing intervals, comprising: operation area specification means operable to specify a setting area (ARe) in a depth direction and a ping direction to be used for the calculation of target-specific information of said target object, said setting area having a shallowest depth and a deepest depth; target information calculation means (41, 42, 43) operable to calculate said target-specific information based on said echo signals; and depth information acquisition means operable to acquire water bottom depth information (Dep) based on said echo signals, characterised in that: the operation area specification means is operable to specify a setting area (ARe) for discriminating a fish species; the apparatus further comprises operation area setting means operable to receive the setting area (ARe) from the operation area specification means and the water bottom depth information (Dep) from the depth information acquisition means, wherein the operation area setting means is operable to perform a process of establishing an application area (RAr) for discriminating a fish species based on the setting area (ARe) specified by said operation area specification means and the water bottom depth information (Dep) acquired by said depth information acquisition means, wherein said application area (RAr) is established based on the presence or absence of said water bottom depth information (Dep) at each said fixed timing interval and based on whether the water bottom depth information (Dep) is between the shallowest depth and deepest depth of the setting area (ARe); and the target information calculation means (41, 42, 43) is operable to discriminate a fish species based on echo signals within said application area (RAr). operation area specification means operable to specify a setting area (ARe) in a depth direction and a ping direction to be used for the calculation of target-specific information of said target object, said setting area having a shallowest depth and a deepest depth; target information calculation means (41, 42, 43) operable to calculate said target-specific information based on said echo signals; and depth information acquisition means operable to acquire water bottom depth information (Dep) based on said echo signals, characterised in that: the operation area specification means is operable to specify a setting area (ARe) for discriminating a fish species; the apparatus further comprises operation area setting means operable to receive the setting area (ARe) from the operation area specification means and the water bottom depth information (Dep) from the depth information acquisition means, wherein the operation area setting means is operable to perform a process of establishing an application area (RAr) for discriminating a fish species based on the setting area (ARe) specified by said operation area specification means and the water bottom depth information (Dep) acquired by said depth information acquisition means, wherein said application area (RAr) is established based on the presence or absence of said water bottom depth information (Dep) at each said fixed timing interval and based on whether the water bottom depth information (Dep) is between the shallowest depth and deepest depth of the setting area (ARe); and the target information calculation means (41, 42, 43) is operable to discriminate a fish species based on echo signals within said application area (RAr). the operation area specification means is operable to specify a setting area (ARe) for discriminating a fish species; the apparatus further comprises operation area setting means operable to receive the setting area (ARe) from the operation area specification means and the water bottom depth information (Dep) from the depth information acquisition means, wherein the operation area setting means is operable to perform a process of establishing an application area (RAr) for discriminating a fish species based on the setting area (ARe) specified by said operation area specification means and the water bottom depth information (Dep) acquired by said depth information acquisition means, wherein said application area (RAr) is established based on the presence or absence of said water bottom depth information (Dep) at each said fixed timing interval and based on whether the water bottom depth information (Dep) is between the shallowest depth and deepest depth of the setting area (ARe); and the target information calculation means (41, 42, 43) is operable to discriminate a fish species based on echo signals within said application area (RAr).