A photographic method to identify benthic assemblages based on demersal trawler discards

• A photographic method to assess trawlers’ benthic discard composition was developed.
• The photographic method was compared to the traditional laboratory method.
• A high correspondence between the two methods was found in habitat classification.
• Benthic assemblages’ spatial distributions were highly consistent in both methods.
• The method allows for the use of demersal trawlers as effective sampling platforms.

Knowledge of the distribution of benthic assemblages is important for monitoring the environmental status of the seafloor and understanding the spatial pattern of demersal fish species and their essential habitats. This knowledge may allow for the enforcement of spatially explicit management approaches, such as those requested by the Marine Strategy Framework Directive. However, traditional methods for benthic fauna sampling are time consuming and expensive, especially when surveying wide areas and relying on expensive sampling platforms, such as research vessels. We developed and tested a photographic method based on mega-epifauna identification and quantification through the analysis of pictures of demersal trawler discards. The method was tested with samples collected in the Northern Adriatic Sea (Mediterranean Sea) at different spatial scales. In this framework, species compositions and abundance were determined through analysis of discard pictures. These samples were compared to those derived from discard samples simultaneously collected during field activities whose specific composition was analysed in the laboratory. The direct comparison between the photographic and laboratory data highlighted a significantly strong correlation in abundance estimates, although the photographic method was less effective for the detection of small-sized or hidden species. The multivariate comparisons of species-site matrices obtained with the two methods also showed a strong, significant correlation, and the spatial patterns of assemblages were significantly consistent. Our results indicate that epifauna discarded by commercial demersal trawlers can be efficiently characterized and quantified using the photographic method, thereby halving the time needed for sample processing and easing practical barriers for sample collection and storage. These data may be used to identify different benthic assemblages and their distributions. This approach could take advantage of ongoing monitoring of commercial fishing activities and/or direct involvement of the fishing industry to allow the collection of benthic species/assemblage data over a wide spatial scale and with a high spatial/temporal resolution, thus making use of fishing vessels as an efficient sampling platform for benthic habitat investigations.