Population dynamics and structure of sperm whales (Physeter macrocephalus) in the Mediterranean Sea

Abstract

The sperm whale (Physeter macrocephalus) has a cosmopolitan distribution. The Mediterranean sperm whale sub-population is classified as ‘Endangered’ according to the International Union for the Conservation of Nature. Even though this population hasn’t faced the whaling age, it is currently threatened by other impacts which may cause direct mortality or stranding (i.e. ship strikes, debris ingestion, entanglement in driftnets, acoustic impacts). Previous genetic, acoustic and markrecapture studies have indicated that this population is distinct from the North Atlantic population. Within the Mediterranean, the population structure is however unclear. Photo-ID data have shown three recaptures between the eastern and the western basins, though acoustic data revealed differences in the coda repertoires of social units between both basins. Our data from the Ligurian Sea to date, where 71 individuals have been identified, suggest that local recaptures are relatively infrequent in the western basin (30 in the Ligurian and 9 in the Tyrrenian Sea) in comparison with data for the eastern basin. Therefore populations may not be fully resident to local areas (based on photo-ID), but there is some evidence for divergence (based on acoustic data). Genetic differentiation among these basins has already been confirmed for various dolphin and fish species. Here, we investigate population structure using ~5000-10000 single nucleotides polymorphism sites (SNPs) generated by restriction-site-associated DNA sequencing (RADseq). We have sequenced 160 samples from stranded and freeranging individuals, 34 samples from the eastern Mediterranean, 82 samples from the western Mediterranean and 44 samples from the eastern North Atlantic. We are analysing these data to address hypotheses about population structure, gene flow, demography, genetic variability and kinship within and among the analysed areas. These results will improve our knowledge of sperm whale demography and patterns of connectivity and are therefore essential for developing a more effective conservation management strategy for this endangered population.