4.3 Implication on species conservation and future perspectives
The ongoing biodiversity crisis is impacting many organisms across the tree of life, and cetaceans are no exception. From the 132 cetacean species, subspecies or populations assessed by the International Union for Conservation of Nature (IUCN), only 51 are considered of least concern, while 24 are considered critically endangered and 24 are classified as endangered. In this study, we have analyzed whole-genome sequencing data of a critically endangered population, the Proper Baltic Sea harbour porpoise, and an endangered subspecies, the Black Sea harbour porpoise. Our results provide genomic evidence that the Proper Baltic Sea population is a distinct lineage (Figure 2, S4) and, given the poor status of the population, urgent measures to protect the species must be implemented (Carlén, Nunny & Simmonds, 2021). Previous studies have shown that microsatellite data does not yield enough statistical power to identify the fine population structure of the harbour porpoise in Baltic waters (Wiemann et al., 2010; Lah et al., 2016). Thus, our results highlight that whole-genome re-sequencing is a powerful tool to unravel even the most subtle population structure. Although in recent years the costs of WGS have been greatly reduced (Fuentes-Pardo & Ruzzante, 2017), it is still prohibitive for small/medium research groups to sequence hundreds or thousands of individuals. In our data, we discovered highly informative SNPs that differentiate the three populations occurring in Baltic waters and which could be used to design a small SNP panel to genotype thousands of samples at a moderate price. Such an approach has been previously implemented in plants (Nygaard et al., 2022), terrestrial (von Thaden et al., 2017) and marine species (Jenkins et al., 2019). With such a panel, bycatch and stranded harbour porpoises could be genotyped and assigned to populations to monitor the conservation status of Baltic porpoise populations.
We identified high levels of inbreeding in the Iberian subspecies, as well as low genetic diversity and Ne compared with the North Atlantic subspecies. Abundance surveys have estimated the Iberian population to around 2,900 animals and have presented one of the lowest population densities on the European continental shelf (Hammond et al., 2013). Previous studies have reported gene flow from Iberia to more northern regions, but not from the North Atlantic subspecies to Iberian porpoises (Ben Chehida et al., 2021). Therefore, following propositions of previous authors (Fontaine, 2014; Fontaine et al., 2016) we confirm the distinctiveness of Iberian porpoises which may warrant subspecies status. Notwithstanding these taxonomic considerations, measures to guarantee the survival of harbour porpoises in Iberian waters are needed. Similarly, Black Sea porpoises presented high levels of inbreeding, low genetic diversity and Ne , which imply that Black Sea porpoises are subject to demographic stochasticity due to strong genetic drift (Palstra & Ruzzante, 2008). Although there are no reliable estimates of current population size across the entire Black Sea, Harbour porpoise mortality in the Black Sea is high, with thousands of animals each year incidentally bycaught (Birkun and Frantzis, 2008; ACCOBAMS, 2020a). In addition, Harbour porpoises and other cetaceans in the Black Sea are highly affected by activities related to fossil fuels extraction, construction work (as the Kerch Bridge), underwater explosions, and different sources of pollution (Carlén, Nunny & Simmonds, 2021). Thus, explicit management policies must be implemented to protect Black Sea Harbour porpoises.