Résumé
The blue shark Prionace glauca, among the most common and widely studied
pelagic sharks, is a top predator, exhibiting the widest distribution
range. However, little is known about its population structure and spatial
dynamics. With an estimated removal of 10 to 20 million individuals per
year by fisheries, the species is classified as “Near Threatened” by
International Union for Conservation of Nature. We lack the knowledge to
forecast the long-term consequences of such a huge removal on this top
predator itself and on its trophic network. The genetic analysis of more
than 200 samples collected at broad scale (from Mediterranean Sea, North
Atlantic and Pacific Oceans) using mtDNA and nine microsatellite markers
allowed to detect signatures of genetic bottlenecks but a nearly complete
genetic homogeneity across the entire studied range. This apparent
panmixia could be explained by a genetic lag-time effect illustrated by
simulations of demographic changes that were not detectable through
standard genetic analysis before a long transitional phase here introduced
as the “population grey zone”. The results presented here can thus
encompass distinct explanatory scenarios spanning from a single
demographic population to several independent populations. This limitation
prevents the genetic-based delineation of stocks and thus the ability to
anticipate the consequences of severe depletions at all scales. More
information is required for the conservation of population(s) and
managements of stocks, which may be provided by large scale sampling not
only of individuals worldwide, but also of loci genome-wide.