Résumé
Two major ecological transitions marked the history of the Black Sea after
the last Ice Age. The first was the postglacial transition from a
brackish-water to a marine ecosystem dominated by porpoises and dolphins,
once this basin was reconnected back to the Mediterranean Sea (ca. 8,000
years B.P.). The second occurred during the last decades, when overfishing
and hunting activities brought these predators close to extinction, deeply
impacting the structure and dynamics of the ecosystem. Estimating the
extent of this decimation is essential for characterizing this
ecosystem's dynamics and for formulating restoration plans. However
this extent is poorly documented in historical records. We addressed this
issue for one of the main Black Sea predators, the harbor porpoise, using
a population genetics approach. Analyzing its genetic diversity using an
Approximate Bayesian Computation approach, we show that only a demographic
expansion (at most 5,000 years ago) followed by a contemporaneous
population collapse can explain the observed genetic data. We demonstrated
that both the postglacial settlement of harbor porpoises in the Black Sea
and the recent anthropogenic activities have left a clear footprint on
their genetic diversity. Specifically, we inferred a strong population
reduction (~90%) that occurred within the last five decades, which can
therefore clearly be related to the recent massive killing of small
cetaceans and to the continuing incidental catches in commercial
fisheries. Our study thus provides a first quantitative assessment of
these demographically catastrophic events, while also showing that two
separate historical events can be inferred from contemporary genetic data.