Trophic niche overlap between sympatric harbour seals (Phoca vitulina)
and grey seals (Halichoerus grypus) at their Southern European limit
range (Eastern English Channel)
Abstract
Competition between the sympatric harbour (Phoca vitulina) and grey
seals (Halichoerus grypus) is thought to underlie some recent local
declines of the former while the population of the latter remains stable
or increases. A better understanding of the interactions between these
two species is critical to elucidate current changes. This study aims at
identifying and quantifying the niche overlap between harbour and grey
seals at their Southern European limit range, in the baie de Somme
(Eastern English Channel, France), in a context of exponential increase
in the number of resident harbour seals and visiting grey seals.
Isotopic niche overlap was quantified between both species using whisker
δ13C and δ15N isotopic values, taking intra- and interindividual
variability into account. Dietary overlap was quantified from scat
contents using hierarchical clustering. A high degree of trophic niche
overlap was identified between both species. The narrower isotopic niche
of harbour seals was nested within that of grey seals (58.2% [CI95%:
22.7-100%] overlap). Six diet clusters were identified from scat
content analysis. Two of them gathered most of harbour seals’ scats
(85.5 % [80.3-90.2%]) and around half of grey seals’ ones (46.8%
[35.1-58.4%]) that almost exclusively contained benthic flatfish.
Consumption of this type of prey was identified here to be the root
cause of trophic overlap. This highlighted the potential for competition
between the two species at their Southern European limit range, linked
to foraging on benthic flatfish, in coastal waters close to their
haulout sites, especially during spring/summer. We suggest that (1)
interspecific competition for prey could occur/increase in the future if
the number of grey and harbour seals still increase and/or if flatfish
supply decrease in this area, and (2) harbour seals would be
disadvantaged in such a case if they do not adapt, as being specialised
on flatfish at the colony scale.