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.