Acoustic discrimination of two zooplankton species (mysid) at 38 and 120 kHz

• Published in: Deep-sea research. Part I, Oceanographic research papers Vol. 46; no. 2; pp. 319 - 333
• Main Authors: David, P.M., Guerin-Ancey, O., Van Cuyck, J.P.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.02.1999; Elsevier; Pergamon Press Inc
• Subjects: Acoustics; Earth, ocean, space; Exact sciences and technology; External geophysics; Frequencies; Hemimysis speluncola; Marine; Marine biology; Marine optics and underwater sound; Microorganisms; Physics of the oceans; Siriella jaltensis; Sonar; Underwater sound; Species diversity; Sonar; Biomass; Measurement method; Acoustic method; Zooplankton; Population density
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/S0967-0637(98)00064-8

Using bifrequency sonar (38 kHz, 120 kHz) directed towards the open sea from the inside of a submarine cave, two species of Mysidacea (Crustacea), similar in shape but not in size, and with different patterns of behaviour, were observed simultaneously and discriminated, and their biomass was estimated during their migration. The first species, Hemimysis speluncola (<1 cm length), lives in the submarine cave, forming a distinct community that migrates horizontally. The second species, Siriella jaltensis (>1 cm length), lives in the open sea and migrates vertically. These species are both detected clearly at 120 kHz, but only the larger one, Siriella jaltensis, is detected at 38 kHz. This distinction allows one to determine the size limits of planktonic objects detected at these frequencies, i.e. 3 mm at 120 kHz and 10 mm at 38 kHz, corresponding to one quarter of the wavelength. The target strength (TS) of each species was calculated from the field data and compared to results calculated from three models. As the volume of the cave occupied by the Hemimysis population is about 300 m 3, and the density of the swarm is 36,000 individuals/m 3, the number of individuals in the cave is estimated to be around 10 million (or 24 kg). The density of the open-water population of Siriella is estimated to be 13 individuals/m 3.