A method for computing volumetric fish density using stereo cameras

Abstract

Cameras are an important tool for sampling marine environments, providing estimates of fish size and abundance for ecological studies and resource management. In addition to length estimates, calibrated stereo-cameras can be used to provide precise locations (i.e. range and angle) of objects within the view frame of both cameras. We present a general method to exploit this information to estimate the density of fishes from stereo-camera measurements. This includes deriving the jointly sampled volume from stereo-camera images, accounting for the presence of the sea floor within the camera view, and accounting for detectability of fishes as a function of range. The method was demonstrated on a set of paired still images collected by stationary camera platforms deployed on the seafloor. Estimates of fish range, camera sampling volume, and seafloor position were used to model volumetric density as a function of range from the camera. A comparison of volumetric density estimates and traditional count metrics found that these metrics differ substantially in inferred relative species composition. The potential value of volumetric density for camera-based marine surveys, the potential for deriving absolute abundance, and the possibility of range-induced bias in count data are discussed.