Harbour seal foraging habitat selection and spatial patterns
Distance from the last haulout, distance from shore and/or bathymetry explained an important part of the deviance (>90%) in harbour seal foraging habitat selection. These three variables had a negative influence, but at different degrees depending on the site configuration. Harbour seals were very coastal and sedentary in the six study areas, which is consistent with previous findings on the species: in the Moray Firth (East of Scotland) for instance, seals forage within 30 Km of their haulout sites and dive at a maximum depth of 50m (Bailey et al., 2014; Tollit et al., 1998). This was also highlighted on the other side of the Atlantic, in the Saint Laurent estuary (Lesage et al., 2004), seals were coastal (with distances shorter than 11 Km from the shore) and in shallow waters (<50m deep). Tidal current and sediment types accounted less in the explained deviance than the other variables. Their influence was generally very low, with the exception of the tidal current for the FoT. Harbour seals feed on diverse fish species and their diet vary locally (Hall et al., 1998). At their southern limit range, in the English Channel, harbour seals mainly selected foraging habitat over mixed sediments in front of estuaries, corresponding to the habitat of juvenile of flatfish that constituted most of harbour seal diet in BdS (Spitz et al., 2015). Seal diet in BdM was not available, we could not compare seals’ foraging habitat selection and their main prey’s habitat there. On the East coast of Scotland, harbour seals selected habitat over sandy areas in front of the Tay river mouth, corresponding to the habitat of sandeel - the main harbour seal prey in the area (Wilson and Hammond, 2019). Harbour seals were more pelagic in the Inner Hebrides, selecting habitat along the sounds of the fjords, corresponding to the habitat of pelagic species such as whiting, scads and herring (Wilson and Hammond, 2019). Sediment and current features selected by harbour seals might correspond to the habitat features of their prey.
Coastal behaviour of harbour seals in the Northeast Atlantic was also identified in their spatial usage for each study area. The median distance from the last haulout was highest for the BdM with 6 Km. However, two patterns were highlighted when performing intraspecific comparisons of spatial distribution of foraging areas for each study sites (BA index). The FoT, Kenmare bay and the study areas in the English Channel showed a high degree of intraspecific spatial overlap (high BA values; ≥0.65). Prey resources might be sufficient to sustain all individuals around these colonies, suggesting a low influence of density dependence. Even though the species is capable of long trips at sea (Lesage et al., 2004), harbour seals in the FoT and Kenmare bay may not need to perform such long trips to find their prey. Conversely, we highlighted a lower degree of intraspecific spatial overlap in the Inner Hebrides (BA value=0.04±0.22). The individual range of trip duration and maximum extent were higher. These results may be explained by the influence of intraspecific interactions, as the colony number in this area is more important than the others (15,200 versus few hundreds, SCOS, 2017). Inter-specific interactions may also have an impact on harbour seals’ spatial usage, as grey seals also breed in the Inner Hebrides (( i.e. 8,700 individuals, Duck et al., 2013; SCOS, 2017; Wilson and Hammond, 2019). This suggests that harbour seals in this area may use a larger area in order to reduce intra- and interspecific indirect competition for resources.
The number of harbour seals at some colonies in the North Sea declined during the last decade (Thompson et al., 2019). The decline of sandeel numbers (main prey of harbour seals around these colonies) and the interspecific competition with grey seals were suggested as potential causes in local harbour seal declines (Wilson and Hammond, 2019). In our study, grey seals and harbour seals were tracked in two areas, where both species haul-out. In this context, it was interesting to compare the two species’ foraging habitat selection and spatial usage. Spatial partitioning between seal species was highlighted in habitat selection and spatial patterns in the FoT and EEC, where both species coexist and were tracked. Grey seals made longer trips than harbour seals (15 Km for median maximum extents in grey seals in both areas, versus 5Km and 4 Km respectively for harbour seals in the EEC and FoT). In both cases, harbour seals tended to forage in inshore areas, while grey seals went further offshore. The two species also selected distinct sediment types. In the EEC, grey seals selected muddy seabed and harbour seals sand, rock and mixed sediments. These findings are in accordance with previous studies, that also found differences in the use of marine environment between these two species (Jones et al., 2015; Sharples et al., 2012).
In the North Atlantic, grey seals and harbours seals are managed at local scales, and in the absence of genetic information on population structuring, haulout groups are often considered as “Seal Management Units” (Russell et al., 2013). Both species are considered as generalist, using a variety of habitat and prey. This study highlights the importance of studying foraging habitat selection at local scale, and considering the variability between colonies, as physical habitat features and seals’ prey resources vary between regions. As marine top predators, both seal species are listed in the Annex II of the European habitat directive requiring establishment of protected areas to maintain favourable conservation status. At a local scale, our predictive maps of foraging habitat selection could be used by managers to implement specific areas of conservation to maintain a good ecological state of their habitat and prey resources potentially at risk due to anthropic activities. Such a foraging habitat selection analysis could be applied and/or adapted for other central place foraging species, in both the marine and terrestrial ecosystems.
ACKNOWLEDGEMENTS
This work was funded by the French ministry in charge of Research (Ministère de l’enseignement supérieur et de la recherche MESR). Seal tracking in France was funded by the Parc naturel marin d’Iroise (PNMI) and the Regional Council of Poitou-Charentes (France), and we acknowledge SMRU, the PNMI, the Office National de la Chasse et de la Faune Sauvage (ONCFS), the Obervatoire PELAGIS, the Zoo de La Fleche, and Oceanopolis for their help in the field. Irish tracking data was funded by Inland Fisheries Ireland (IFI); Department of Communications, Marine and Natural Resources (DCMNR); Higher Education Authority of Ireland (HEA); National Parks and Wildlife Service (NPWS), and National Geographic Global Exploration Fund. We would like to thanks the Sea Mammal Research Unit for providing telemetry data for grey seals and harbour seals in UK. Seals were caught under licences issued by the French ministry of the environment for France (Licences Nos : 01/161/AUT, 01/525/AUT, 03/380/AUT, 05/475/AUT, 05/485/AUT, 06/82/AUT, 07/481/AUT, 08/346/DEROG, 08/347/DEROG, 10/102/DEROG, 11/873/DEROG,11/874/DEROG, and 13/422/DEROG.); by the National Parks & Wildlife Service (Licence Nos: C35/2008, C014/2012, C0019/2011, C04/C023/2013, and C016/2014), the Irish Health Products Regulatory Authority for Ireland (Project Licence AE19130/P004); licence provided by Uk home office under the Animals (Scientific Procedure) Act 1986 and the Scottish Executive under the Conservation of Seals Act for Uk.1970.
AUTHORS’ CONTRIBUTION
MH and CV conceived the ideas and developed the methodology with the help of YP. CV, MC and MJ collected the data. MH analysed the data and led the writing of the manuscript. All authors contributed critically to the drafts and have final approval for publication.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study will be available on GitHub as manuscript will be accepted.