The significance of stranding data as indicators of cetacean populations at sea: Modelling the drift of cetacean carcasses

Stranded marine mammals are an important source of information and biological samples on cetacean population. Nevertheless, collecting stranding data remains opportunistic and its representativity must be improved, both qualitatively and quantitatively. Drifts of small cetaceans found by-caught in fishery observation projects and subsequently released dead with a numbered tag fitted to the tail fluke were predicted by using the Météo-France drift model MOTHY and allowed us to assess the proportion of dead dolphins recovered by volunteers of the French stranding network. Only 8% of dolphins were recovered ashore. The spatial representativity of strandings was assessed by performing back-calculation of carcass drift with the same model in order to map the likely origin of stranded cetaceans. As a first step, external visual criteria of time-after-death (as a proxy to drift duration) were obtained from series of photographs of dead small cetaceans maintained in a floating cage for 40 days and from tagged by-caught dolphins recovered stranded after a drift in real condition. Then, pictures of 242 stranded common dolphins (Delphinus delphis) were used to establish the average distribution of dolphin time-after-death in this area. Finally, 40 days-long reverse drifts of the 829 common dolphins recorded in the winter months of 2004–2009 were weighted by the modelled distribution of time-after-death in order to map the areas of likely origin. It appeared that most stranded common dolphins recorded along the French Atlantic coast originated from the continental shelf, mostly in the south of the Bay of Biscay. These results open new perspective on the use of stranding data and biological samples as sources of indicators in monitoring strategies.