Lateral aspect acoustic frequency response of gas bubble plumes

Laterally observing fisheries echosounders are useful in specific applications, such as to inspect surface blind zones from a vessel, to observe wild fish gathering under fish-farm nets or for bottom-mounted acoustic landers with both vertical and horizontal acoustic beams. The frequency response (r(f)) measured from the dorsal aspect is frequently used to identify species, but it is not clear if dorsally measured r(f) is useful for identifying laterally observed fish. The r(f) of free-swimming wild saithe was measured with stationary and laterally oriented split-beam echosounders at 70, 120, 200, 333 kHz, suspended in mid-water (22 m, bottom depth 41 m).

Simultaneously recorded imagery (video and stereo-photo) were used for species identification and sizing. Fish aggregating close to a small fish-farm were observed. Saithe of 45-55 cm in length was observed in small schools of varying density and recorded at 5-35 m range while swimming in and out of the acoustic observation volume. The SV(f)-based, TS(f)-based and fish track average TSTrack(f)-based acoustic frequency responses were derived. Lateral saithe r(f) increased smoothly with frequency, which is the opposite trend to dorsal aspect r(f) reported in the literature.

Presentation at “SOMEACOUSTICS” ICES Symposium on Marine Ecosystem Acoustics, 2015