OASIS in the sea: measurement of the acoustic reflectivity of zooplankton with concurrent optical imaging

A new instrument Optical-Acoustic Submersible Imaging System (OASIS) has be en developed for three-dimensional acoustic tracking of zooplankton with co ncurrent optical imaging to verify the identity of the insonified organisms . OASIS also measures in situ target strengths (TS) of freely swimming zoop lankton and nekton of known identity and 3-D orientation. The system consis ts of a three-dimensional acoustic imaging system (FishTV), a sensitive opt ical CCD camera with red-filtered strobe illumination, and ancillary oceano graphic sensors. The sonar triggers the acquisition of an optical image whe n it detects the presence of a significant target in the precise location w here the camera, strobe and sonar are co-registered. Acoustic TS can then b e related to the optical image, which permits identification of the animal and its 3-D aspect. The system was recently deployed (August 1996) in Saanich Inlet, B.C., Cana da. Motile zooplankton and nekton were imaged with no evidence of reaction to or avoidance of the OASIS instrument package. Target strengths of many a coustic reflectors were recorded in parallel with the optical images, trigg ered by the presence of an animal in the correct location of the sonar syst em. Inspection of the optical images, corroborated with zooplankton samplin g with a MOCNESS net, revealed that the joint optically and acoustically se nsed taxa at the site were the euphausiid Euphausia pacifica, the gammarid amphipod Orchomene obtusa, and a gadid fish. The simultaneous optical and a coustic images permitted an exact correlation of TS and taxa. Computer simu lations from a model of the backscattered strength from euphausiids are in good agreement with the observed data. (C) 1998 Elsevier Science Ltd. All r ights reserved.