Abstract:

A new instrument has recently been developed: OASIS (optical-acoustical submersible imaging system), for recording in-situ acoustic target strengths (TS) of crustacean zooplankton with concurrent optical imaging of the animals. The system consists of a three-dimensional acoustic imaging system (Fish TV) and a sensitive optical CCD camera with a red-filtered strobe. The sonar triggers the acquisition of an optical image when it detects the presence of a significant target in the precise location where the camera and strobe are co-registered. Acoustic TS can then be related to the optical image which permits the identification of the animal and its 3-D orientation. The system was deployed in Saanich Inlet, BC, Canada in the summers of '86 and '87. Target strengths of many acoustic reflectors were recorded in parallel with the optical images triggered by the presence of an animal in the correct location of the sonar system. Inspection of the optical images revealed that the primary zooplankton taxa at the site were euphausiids [esp. Euphausia pacifica] and [gammarid] amphipods [esp. Cyphocaris challengeri] and permitted an exact correlation of TS and taxa. Extrapolation of this information to many other acoustic targets, imaged by the Fish TV system will allow more precise interpretation of both abundance and behavior.