Visual observations were collected using video from a remotely, operated vehicle (ROV) for a number of deep-water species on the continental slope of the Bay of Biscay, !Northeast Atlantic. Relative trawl availability was defined as the ratio of population density estimates from a scientific bottom-trawl survey to those derived from visual strip transects. Several natural and reaction behaviour variables were explored to explain between-species variation in relative trawl availabitility. The variable with most explanatory power was spatial dispersion, with aggregating species showing lower relative availability than those that were randomly or uniformly distributed. This variable,was also strongly correlated with ROV density estimates (aggregated species had the highest densities.). Mean distance off the ground and mean body length of a species were positively related to relative trawl availability. In contrast, the way different species reacted to the ROV did not appear to be correlated with relative trawl availability.