Résumé
Harbour porpoises (Phocoena phocoena) and harbour seals (Phoca vitulina) are far
more abundant along our coast compared to the beginning of the nineties. Human
impact on these species is however hard to establish, mainly due to lack of
information on marine mammal population ecology, density, distribution and diversity.
This project aims to gain further knowledge on the viability of the harbour porpoise
and harbour seal populations in the North Sea (focusing mainly on its southern Bight)
through
- The characterisation of their genetic structure and diversity (through mtDNA and
microsatellites in harbour porpoises)
- A better understanding of their feeding ecology (through δ13C and δ15N
measurements in muscles)
- The assessment of their susceptibility of being trapped accidentally in fishing
nets (post-mortem investigations)
Harbour porpoise and harbour seal occupied the top trophic levels but displayed
different feeding habits as inferred from their δ13C and δ15N mean values. Harbour
porpoises displayed lower mean δ15N values suggesting a lower trophic position
likely oriented towards small planktivorous fish such as herring and lesser sandeel.
However, both their recent high abundance and their dietary preferences might lead
to a higher susceptibility to by-catch as revealed by the significant emergence of net
entrapment and net marks revealed by post-mortem investigations. The question
rises about the sustainability of these incidental captures. Furthermore, genetic
investigations revealed a higher fragmentation of the porpoises collected along the
coast of France, Belgium and Netherlands. This apparent fragmentation is of
particular importance from a conservation point of view and enhances the fact to
protect in priority these last populations.
Our study showed importance of multidisciplinary approaches (post-mortem
investigations, stable isotope measurements (δ13C and δ15N measurements) and
genetic investigations using mtDNA and microsatellites) to apprehend the question of
marine mammal survival in our waters.