Underwater television for fisheries-independent stock assessment of Nephrops norvegicus from the Aegean (eastern Mediterranean) Sea

The underwater television (UWTV) technique for Nephrops stock assessment was evaluated from 7 grounds in the Aegean. Expert analysts made all burrow identification and burrow size measurements. Burrow density was given by (1) real-time in situ counts; (2) post-sampling laboratory replicate counts; and (3) multiple laboratory counts in strips analyzed by DISTANCE sampling in order to account for partial burrows and edge effects leading to over- or underestimation of densities. Trawls were used to estimate densities, mean carapace length and weight per ground. The laboratory method was the most efficient approach. In situ counts resulted in underestimation at high densities and overestimation at low densities. Further work with DISTANCE should include simulations with hypothetical data to optimise band widths related to the expected burrow size. All burrow density estimates produced similar significant negative relationships with burrow size and positive relationships with trawl density. Based on the tow length effects analysis, the use of tow lengths of 500 and 1000 m per station for high and mid-/low-density sites, respectively, is recommended. With expert analysts, laboratory counts, trawl support data, accurate delimitation of the grounds and adequate sampling spread to allow for local stock variability differences, the UWTV technique can be used for fishery-independent stock assessments, and its precision refined by taking into account edge effects.