Exploring cetacean stranding pattern in light of variation in at-sea encounter rate and fishing activity: Lessons from time surveys in the south Bay of Biscay (East-Atlantic; France)

To date, the scarcity of year-round and long-term programmes integrating multi-dimensional data has hindered the development of a good understanding of cetacean mortality worldwide. This study uses data from: 1) standardised shipboard surveys (1980–2002), 2) standardised stranding surveys (1980–2002) and 3) landings of fishing fleets (2000–2002) in the Bay of Biscay. It investigates the correlations between stranding, at-sea encounter rate and the fishing index for three common cetaceans: common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus) and long-finned pilot whale (Globicephala melas). At the monthly scale, a seasonal stranding pattern significantly congruent with the at-sea encounter rate and the fishing index for D. delphis is revealed. At the inter-annual scale, stranding and at-sea encounter rates are shown to be correlated (p=0.013–0.044 according to species) and significantly increasing in abundance. Temporal variation in the ratio between individuals seen alive at sea and those stranded shows no significant trend suggesting that stranding is better explained by at-sea abundance than by the fishing index. Managers can use these findings to re-evaluate the relative contribution of by-catch fisheries in the context of observed changes of at-sea cetacean abundance and the link to oceano-climatic changes and other anthropogenic causes.