Many kinds of fish, including croaker, produce species-specific low-frequency sounds associated with courtship and spawning. A recording system was used to monitor underwater fish call sounds. A method was previously proposed to detect croaker calls from data measured by a single hydrophone (channel). However, it was difficult to detect the desired calls at a high detection rate because of a low signal-to-noise ratio and overlapping of calls. In this manuscript, we propose a new method using beamforming to improve the detection rate. The proposed method uses four-channel data recorded by two stereo recorders. First, calls are detected from data measured on one channel(ch1) using the previous method, which detects calls automatically using the acoustic features of fish calls such as duration and periodicity. Next, additional calls are detected by beamforming the four-channel data. The detection rate using the proposed beamforming method was higher than that under the previous method with a single data channel, indicating that the proposed method is able to monitor sounds from croaker in the ocean.