Dear MARMAM colleagues,

We are pleased to share our new article published in Ecology and Evolution:

Planque Y, Spitz J, Authier M, Guillou G, Vincent C, Caurant F. 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). Ecol Evol. 2021;00:1– 22. (Abstract below)

The article is freely available online from https://doi.org/10.1002/ece3.7739 (Open Access).

For people interested by the analyses used in this study, all materials (data and R scripts) are publicly accessible from these links:

→ scat contents data (https://doi.org/10.17882/76780) used for diet analysis (diet clusters and overlap) (https://github.com/YannPlanque/Diet_Cluster_and_Overlap)

→ stable isotopes data (https://doi.org/10.17882/76528) used to identify isotopic niches with a novel hierarchical Bayesian model considering both intra- and interindividual variability (https://github.com/YannPlanque/Isotopic_Niche_Overlap)

→ dive data from telemetry tracking (https://doi.org/10.17882/80016) used to identify foraging areas (https://github.com/YannPlanque/Foraging_Areas_with_Dive_Data)

All the best,

Yann Planque and co-authors

Abstract: 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.