In situ measurements of acoustic target strengths of gas-bearing siphonophores

Acoustic target strengths of free-swinnning siphonophores were measured in situ at 24 and 120 kHz from a remotely operated vehicle equipped with both acoustic transducers and a video camera. The transducers and camera were co-registered by aiming both instruments at the same volume of water and time-stamping the recorded data. The video system allowed us to search for and identify siphonophores, and verified whether individual animals were centered in, or near, the axis of the acoustic beams. A towed, down-looking acoustic survey system (operating at 120 kHz) measured the target and volume scattering strengths of scattering layers, presumed to be dominated by siphonophores. Spatial density of the sound scatterers was estimated from survey data. Our results confirm that free-swimming physonect siphonophores have relatively high acoustical target strengths caused by a gas inclusion in the pneumatophore of each animal. A relatively small number of these animals can dominate the backscattering detected by acoustic surveys even though other taxa may dominate the plankton on a numerical or biomass basis. Siphonophore colonies are fragile and cannot be reliably censused with nets. Our estimates of siphonophore target strengths can improve the ability to use acoustics to quantitatively census siphonophores and other taxa possessing comparably-sized gas inclusions.