Trophic niche overlap between sympatric harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) at the southern limit of their European range (Eastern English Channel)
Résumé
Sympatric harbour (Phoca vitulina) and grey seals (Halichoerus grypus) are increasingly
considered potential competitors, especially since recent local declines in harbour
seal numbers while grey seal numbers remained stable or increased at their European
core distributions. A better understanding of the interactions between these species
is critical for conservation efforts. This study aimed to identify the trophic niche
overlap between harbour and grey seals at the southern limit of their European
range, in the Baie de Somme (BDS, Eastern English Channel, France), where numbers
of resident harbour seals and visiting grey seals are increasing exponentially. Dietary
overlap was identified from scat contents using hierarchical clustering. Isotopic niche
overlap was quantified using δ13C and δ15N isotopic values from whiskers of 18 individuals,
by estimating isotopic standard ellipses with a novel hierarchical model
developed in a Bayesian framework to consider both intraindividual variability and
interindividual variability. Foraging areas of these individuals were identified from
telemetry data. The three independent approaches provided converging results, revealing
a high trophic niche overlap due to consumption of benthic flatfish. Two diet
clusters were dominated by either small or large benthic flatfish; these comprised
85.5% [CI95%: 80.3%–90.2%]
of harbour seal scats and 46.8% [35.1%–58.4%]
of grey
seal scats. The narrower isotopic niche of harbour seals was nested within that of
grey seals (58.2% [22.7%–100%]
overlap). Grey seals with isotopic values similar to
harbour seals foraged in coastal waters close to the BDS alike harbour seals did, suggesting
the niche overlap may be due to individual grey seal strategies. Our findings
therefore provide the basis for potential competition between both species (foraging
on benthic flatfish close to the BDS). We suggest that a continued increase in seal
numbers and/or a decrease in flatfish supply in this area could cause/amplify competitive
interactions and have deleterious effects on harbour seal colonies.
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