Stables isotopes in the fin whale Balaenoptera physalus from the Mediterranean Sea: implications for management and conservation

The fin whale (Balaenoptera physalus) is the commonest large whale species in the Mediterranean Sea, found mostly over deep, offshore waters of the western and central portion of the region. This whale is known to feed mainly on krill in contrast to its Atlantic counterpart, displaying a more diversified diet. δ13C and δ15N values were analysed by IR-MS (Isoprime 100) coupled to an N-C-S elemental analyser (Vario MICRO Cube, Elementar) in 113 skin biopsies from Mediterranean fin whales sampled in 2010 and 2011 during WWF campaign at sea. A lipid normalization equation was applied (adapted from Post al. 2007) because the measured C:N ratio was > 3.5. Normalized δ13C and δ15N values ranged from -20.3 to -17.3 ‰ and from 5.9 to 8.9 ‰, respectively. These values are in good agreement with those collected previously on baleen plates from Mediterranean fin whales (Bentaleb et al., 2011). A mean enrichment of 3.4 ‰ was observed between δ15N values measured in fin whale skin biopsies and Mediterranean krill (Meganyctiphanes norvegica) confirming the importance of the krill as a major food source. The narrow width of the isotopic niche of the Mediterranean fin whale (evaluated by SIBER Stable Isotope Bayesian Ellipses) compared to the Atlantic fin whale (Ryan et al. 2012) raises many concerns in the context of global changes and long-term consequences. One could expect that species displaying narrow niches would be more susceptible to ecosystem fragmentation and other anthropogenic impacts.