Fishery-independent estimation of benthic species density-a novel approach applied to Norway lobster Nephrops norvegicus

The use of image-based technologies has gained increasing importance in marine research, particularly in benthic habitat classification and the assessment of epibenthic commercial species. A common application that has now entered into the routine of a number of northeastern Atlantic research institutes and in the Mediterranean (on an irregular basis) is fishery-independent estimation of Norway lobster Nephrops norvegicus abundance. Typically, the experimental set-up consists of a video camera mounted on a sledge towed from a research vessel during targeted surveys. Targeted surveys produce superior images; however, fishing operations cannot be carried out simultaneously, which limits the amount of information collected. During a research cruise testing the use of by-catch reduction devices (BRD) to optimise the fishing pattern of Norway lobster, an underwater video camera was hung from the trawl headline, angled down in the tow direction to register ground images. We evaluated the potential usefulness of this set-up as an alternative to traditional methods for the study of benthic habitats and present a case study of the Norway lobster. A preliminary estimate of burrow entrance density (2.1 m(-2)) in Portuguese waters was obtained by superimposing a measuring grid over individual image frames. The combined approach (video recording while trawling) also allowed a first estimate of the survey trawl efficiency (0.46). The advantages (e.g. simultaneous image and lobster samples) and disadvantages (e.g. low quality image due to the high trawling speed and mud clouds) of the experimental set-up are discussed, concluding by its practicability. However, its application to Norway lobster density estimation as an alternative to current procedures will require further development.