Foraging ecology of Mediterranean fin whales in a changing environment elucidated by satellite tracking and baleen plate stable isotopes

We investigated seasonal shifts in diet and distribution of fin whales Balaenoptera physalus occurring in the western Mediterranean Sea. For this purpose, we combined carbon and nitrogen stable isotope ratios (delta C-13, delta N-15) along 10 baleen plates collected from stranded fin whales between 1975 and 2002 with satellite tag deployments on 11 fin whales during summer 2003. Baleen plate stable isotopes were compared with those of the krill Meganyctiphanes norvegica, the main prey of fin whales in the northwestern Mediterranean Sea. Two plates collected near Malaga, Spain, exhibited larger delta C-13 variations, while only smaller variations could be detected in the other 8. While all mean baleen plate results were consistent with the delta C-13 signature of Mediterranean M. norvegica, the most depleted delta C-13 values were intermediate between those of Atlantic and Mediterranean M. norvegica, suggesting westward migrations perhaps extending to the Strait of Gibraltar but not extensive, prolonged feeding in the Northeast Atlantic. This pattern was confirmed by satellite tracking; 1 out of 8 fin whales we successfully tracked left the Mediterranean for the Atlantic. Longer-term changes in isotopic signatures of baleen plates exhibited significant depletion trends, indicating that changes due to increasing input of nutrients and anthropogenic carbon are occurring in the western Mediterranean Sea ecosystem.