Applying Stable Isotopes Bayesian Ellipses (SIBER) to characterise trophic niches of large cetaceans from the north-Western Medterranean Sea

Diet composition and feeding preferences are of critical importance to understand a species' ecology; better knowledge of these matters is necessary for efficient conservation. Few is known on the trophic ecology of Mediterranean cetacean populations and the elusiveness of these organisms limits acquisition of new information. Here, we analysed C and N isotopic ratios of skin biopsies of 17 fin whales Balaenoptera physalus, 15 long-finned pilot whales Globicephala melas and 25 sperm whales Physeter macrocephalus sampled through darting between summer 2010 and 2013 in the North-Western Mediterranean Sea. We subsequently used the SIBER R package to explore isotopic niche parameters as a proxy for trophic niches assessment. Fitting of standard ellipses to each species revealed that no niche overlap between the odontocetes and fin whales was present, in accordance with the lower trophic level of the latter. Moreover, overlap between the isotopic niches of the two odontocetes was limited, confirming resource partitioning between these two species. This could be linked with differences in hunting periods and depths and consequently in prey availability. Bayesian modelling of standard ellipses revealed that the isotopic niche of fin whales was larger than the two odontocetes in over 99.80% of 106 model simulations. It is the first time that such variability is observed in Mediterranean fin whales. This suggests possible exploitation of food items from different trophic levels (krill, small fishes) or from other regions, such as other areas in the Mediterranean Sea and the North-East Atlantic. Modelling also suggested that no meaningful differences were present in the width of isotopic niches of the two odontocetes. The very narrow isotopic niche of pilot whales is in strong contrast with the generalist feeding behaviour this species is believed to have according to literature analysis. Our results open new perspectives on the ecological role of Mediterranean cetaceans.