Eco-Phoques : Bases scientifiques pour une meilleure connaissance des phoques et de leurs interactions avec les activités humaines en Manche Nord-Est: Rapport final pour la région Hauts-de-France et l’AFB.

Brévart, Clémentine, Leviez, Frédéric, Planque, Yann, Anglade, Manon, Karpouzopoulos, Jacky, Frémau, Marie-Hélène, Dupuis, Laetitia, Decomble, Mélia, Mestre, Julie, Martin, Christine, Lemaire, Daniel, Varin, Corinne, Delcourt, Régis, Everard, Aymeric, Lamour, A, Gosselin, Damien, Dehove, M, Hochard, Muriel, Thiery, P, Bourgain, Jean-Luc, Dubois, Aurélie, Noël, Charlotte, Rault, Célia, Brabant, Claire, Mahieux, Justine, Savoie, J, Magniez, Alicia, Decroix, A, Gobillot, Julie, Devos, J, Pillerel, C, E, Lahaye, Wimez, Kévin, Kerouedan, F and Vincent, Cécile (2018) Eco-Phoques : Bases scientifiques pour une meilleure connaissance des phoques et de leurs interactions avec les activités humaines en Manche Nord-Est: Rapport final pour la région Hauts-de-France et l’AFB. 127pp.

Abstract

Two seal species are found year-round in the Région Hauts-de-France (North of France): the harbour seal (Phoca vitulina), observed for decades in the area and still increasing, and the grey seal (Halicheorus grypus), more recently settled, which numbers have increased exponentially this last decade. While harbour seals are observed at all seasons and seem to be very faithful to their haulout sites, grey seals, mostly adult males, are mainly abundant in the area seasonally (during summer). More harbour seals are also counted on their haulout sites during summer, but this is most likely only due to their spending more time ashore during the breeding and moulting seasons. The maximum number of seals simultaneously censused on the haulout sites from baie de Somme to Dunkerque was 709 harbour seals in July 2017, and 402 grey seals in July 2016. These maxima increased these last years, even if in some haulout areas the yearly maximum counts remain stable since 2015. Telemetry tracking showed that harbour seals from the baie de Somme make very coastal movements, and forage mostly in front of (or close vicinity, a few kilometres away from) the baie de Somme. They can travel over a few tens or a hundred of kilometres, sometimes moving from one colony to another, as also shown by photo-identification matching between the baie d’Authie and baie de Canche. However, telemetry showed that grey seals move at a much larger scale, travelling over tens and hundreds of kilometres, foraging in the middle of the English Channel, in baie de Somme or along the coasts of Hauts-de-France, sometimes further in the North Sea where they return for breeding. Photo-identification also highlighted individual movements between baie de Somme, baie d’Authie and phare de Walde. This technique showed that seals from both species were faithful to their haulout sites. This site fidelity was higher in harbour seals, but a significant proportion of grey seals also came back from one year to the next. The haulout site at phare de Walde seems to be a step-over area for grey seals (from the Southern North Sea), with more photoidentified individuals observed only once.

The seals’ diet, estimated from faeces sampled in baie de Somme (and also Walde for grey seals) consisted of 86% [95% confidence interval: 68-100%, in biomass] of flat fish and 14% [5-25%] of round fish for harbour seals, and of 61% [38-87%] of flat fish, 29% [13-54%] of round fish and 10% [1-23%] of cephalopods for grey seals. In both seal species, flat fish species were mainly plaice, flounder or different sole species, and the main round fish species in grey seals was herring. Additional diet analysis from stomach samples of stranded seals suggested that cephalopods may be underestimated in the grey seal diet especially. Trophic overlap between both seal species was high, but a higher inter-individual variability was observed in grey seals, considered as individual specialists, than in harbour seals, rather considered as specialized at the colony level. The analysis of ecological tracers (carbon and nitrogen stable isotopes) showed that results of seal diet obtained exclusively in baie de Somme cannot always be generalized to the whole region, and that the trophic niche of harbour seals was included in that of grey seals. All these results (on diet and tracers) suggest interactions between both species, and potentially competition, that could evolve in the future if grey seal numbers continue to grow. From 2015 to 2017, total prey consumption by seals from baie de Somme and baie d’Authie increased from 592 [490-701] to 750 [632-878] tons per year for harbour seals and from 422 [369-476] to 750 [675-827] tons per year for grey seals. Seasonal variations in total consumption were limited for harbour seals, but larger for grey seals, mostly abundant in the area during summer. During this summer season, their prey consumption most likely exceeds that of harbour seals.

Both seal species are strongly interacting with each other, but less so with fishery activities, except for trammelers for which the fishing effort strongly overlaps with harbour seal foraging areas (overlap 18 times higher between trammelers and harbour seals than between all fishing gear and harbour seals). Sport and tourism activities also cause interactions with seals, that are either seen as positive (e.g. seals attract tourism, contribute to the regional identity…) or negative (risks of disturbance of the seals). Users along the coast are conscious of these interactions, and it seems necessary to maintain the existing monitoring in order to better evaluate those interactions, in a context of increasing seal populations.

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Contributors

Author: Julie Mestre

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