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.