Late night fish choruses in the shallow waters off San Diego

Abstract

Two major experiments, SWellEx-1 and SWellEx-3, and one engineering sea test recently have been conducted in 200-m-deep water 8 miles west of the mouth of San Diego harbor. SWellEx-1 was conducted in August of 1993, the engineering sea test in May 1994, and SWellEx-3 in July 1994. For both of the summertime SWellEx experiments, an unusual oscillation in the background levels between 275–550 Hz and 575–700 Hz was observed, typically starting around midnight local time and lasting until sunrise. The repeating pattern to the oscillations is that of a 3–5 dB increase in spectral levels, which lasts on the order of 20 s, and then followed by 25 s or so of lower levels. Azimuthal beamforming has indicated that the predominant direction of this ‘‘cycling sound’’ is from near coastal waters. In the data from the May 1994 engineering sea test, no such cycling sound was present. The character of these signals, i.e., their spectral content, their diurnal variation, their seasonal variation, and their directionality, correspond to those described in year-long, single-hydrophone measurements made from a shallow water tower off San Diego in the early 1960s and ascribed to sounds made by fish of the croaker family [G. A. Clapp, NEL Tech Mem 1027 (1966)]. Additional properties of these signals, including a recording of the sounds made by a single individual, will be presented. [Work supported by ONR Code 321.]