Estimating the density of thornyheads, Sebastolobus spp., using a towed video camera sled

A video camera sled was used to obtain an independent estimate of the density of Sebastolobus spp. (thornyheads) at three different depths (450, 750, and 1150 m) within a given space and time. Camera sled video footage was processed using an oblique grid plane and line transect methods. Thornyheads were randomly distributed across the seafloor within the sampling area, and variation in the dispersion over increasing spatial scales (10–1280 m) and depths was not significant. Thornyhead density estimates were derived using the program Distance. Densities ranged from 344 to 1005 thornyheads km −2 and CVs from 10 to 18%. Underlying assumptions necessary for obtaining unbiased density estimates using a camera sled and line transect sampling are discussed, including a methodology to account for fish movement in response to the camera sled.