Use of a video and laser system to quantify transect area for remotely operated vehicle (ROV) rockfish and abalone surveys

In situ surveys by remotely operated vehicle (ROV) and submersibles are important sampling tools for making fish and invertebrate population estimates. A major challenge associated with these methods is the measurement of the search and/or transect area due to variation in the field of view (FOV) throughout a dive. In addition, to effectively survey complex habitats it is often desirable to adjust the camera angle, which results in significant differences in the FOV in the recorded images and subsequent spatially variant image magnification. This paper describes a method for obtaining accurate surveyed area estimates using FOV measurements derived from image analysis and from the speed of the vehicle measured by a Doppler velocity logger (DVL). The image analysis software considers camera tilt angle, roll and pitch of the ROV, distance from the bottom, and the location of the reference lasers in the imagery to calculate the field of view. The DVL continuously logs forward/aft and port/starboard speeds for a representation of the vehicles movement over the bottom, and in effect, distance traveled. The development of these methods has led to a highly accurate estimate of transect area that will be used to calculate densities of commercially important rockfishes and endangered white abalone in the Southern California Bight.