Estimation d’abondance de la raie brunette dans le Golfe de Gascogne par marquage-recapture

An undulate ray (Raja undulata) tagging survey was carried out from the end of 2011 to mid-2014 in the Bay of Biscay with the partnership of the fishing industry. It demonstrates that the undulate ray can be found all along the French coast from the Loire estuary to the Spanish boarder, forming several isolated units, the more important being likely in the central part of the Bay of Biscay (Pertuis Charentais – Gironde area). Even in this latter limited area, the population is structured in sub-units with a low exchange rate between them. This population structure allowed to estimate abundance by mark-recapture in the Gironde estuary, using a Petersen estimate. The conditions that must be respected for such closed population abundance estimate are analysed. The conclusions are that as long i) long-line catch of rays longer than 65 cm are used to the be sure to have an equal capture probability and no recruitment effect, ii) recaptures are within 4.5 month from the tagging date in winter to neglect tag losses and iii) number of tagged rays is corrected for emigration and mortality, an abundance estimate can be provided. The biomass of undulate rays longer than 65 cm in the inner Gironde estuary can thus be estimated to range from 51 to 70 t in the 2013-2014 winter (95% confidence interval is 30-124 t). This first trial allows to have some guidelines for future mark-recapture estimates of the abundance of a species for which the use of other methods may be difficult.