Periodic Variations of the Underwater Ambient‐Noise Level of Biological Origin

Two distinct periodic variations in the underwater noise level have been observed in coastal waters off San Diego. The first has been previously reported and appears diurnally before sunset, reaches maximum amplitude just after sunset, and then fades away within 3 h. This variation is observed in the 100‐ to 1000‐cps band and raises the sound‐pressure level 5–20 dB above the normal background, with maximum duration and intensity occurring in the Summer months. This effect is occasionally noted at sunrise, though with lower amplitude than at local sunset. The second periodic variation has a period of 25–40 sec and is observed only at night in the late Spring and Summer months. Its amplitude is commonly from 3 to 6 dB but maximums of 16 dB have been noted. It is observed above the background in the 300‐ to 800‐cps band, with peak amplitude occurring at about 450 cps. Two members of the croaker family—spotfin, Roncador stearnsi, and yellowfin, Umbrina roncador—are in part responsible for the periodic variations.