This paper examines the differences in the features of sounds produced by yellowtails and amberjacks of different size during swimming and bait-eating activities in a net pen. Recorded underwater sounds were analyzed by the one-third octave band method.
The overall underwater ambient noise levels in the Tarumizu and Furue waters were 108.6 dB and 106.4 dB, respectively. The sound levels of yellowtail were 102.8-116.0 dB during swimming and 113.4-124.8 dB during bait-eating. The sound levels of amberjack were 109.4-110.2 dB in the swimming activity and 115.3-121.9 dB in the bait-eating activity. The sound level in the bait-eating activity was higher than that in the swimming.
Graphs of one-third octave band analysis of the underwater sounds in the two activities could be classified in four specific patterns. The curve patterns indicated some relationship with sizes of the fish. Frequencies at the minimum sound level in a concave part of the pattern and at the maximum level in the swimming and bait-eating activities were higher for the small-sized fish and vice versa. Both frequencies of the amberjack sounds were lower than those of the yellowtail sounds in the respective sizes and activities.