Grey seal foraging habitat selection and spatial patterns
The distance from the last haulout was the most important factor
influencing the foraging habitat selection of grey seals in all study
areas (from 45% to 76% of the explained deviance in the model
respectively for the FoT and the Iroise Sea). Our data shows that when
performing return trips, they spend the majority of their time close to
the haulout sites. These results are consistent with previous studies
throughout the range of grey seals in the Northeast Atlantic. Grey seals
in the North Sea spent 43% of their time within 10Km of the haulout
sites (McConnell et al., 1999) and preferentially selected habitat
closer to haulout-sites with a gradual decrease of habitat selection
beyond tens of kilometres (Aarts et al., 2008). In the Baltic Sea,
Sjöberg and Ball, (2000) noted short distance trips (from 10 to 15 Km),
grey seals spending 75% of their time within a radius smaller than 50Km
around their haulout sites. In the Iroise Sea, grey seals mostly
undertook trips within 40Km of haulout sites (Huon et al., 2015).
Foraging habitat selection by grey seals was negatively influenced by
bathymetry, but to a lesser extent than distance from haulout sites
(varying from 10 to 40% in the explained deviance). Depending on study
sites, the depth selection decreased until a depth of 50m (Irish and
Iroise Sea) and 100m (FoT). These results are consistent with previous
studies on grey seals habitat selection, with usual dive depth between
10 to 80 m (Aarts et al., 2008; Tollit et al., 1998). Grey seals are
generally considered as benthic feeders (Beck et al., 2003; Lydersen et
al., 1994) and the influence of bathymetry on foraging habitat selection
will presumably vary locally depending on the seabed topography and
sediment type. Furthermore, distance from the shore and tidal current
must also influence the behaviour of seals, as they use them to
orientate, to move, and to forage (Zamon, 2003, 2001). These three
variables did not contribute much to the explained deviance of the
models, and their influence varied among sites. Distribution of seals’
prey resources was not included in the model, as the data were not
available for some of the study areas.
Grey seals in the Firth of Tay and in the Eastern English Channel made
longer trips but of shorter duration compared to grey seals in the other
sites. In the Northeast Atlantic, the core population is in the North
Sea with 141,000 individuals (SCOS, 2017). In this area, due to the
important pressure on prey resources located in inshore areas, grey
seals tend to travel long distances directly to offshore areas on
specific sandbanks where sandeel availability is high (Hammond et al.,
1994; McConnell et al., 1999; Wilson and Hammond, 2019). It may be a
process to reduce the effect of density dependence and to avoid the
indirect competition for resources with fisheries. This offshore
behaviour is in accordance with our results for the FoT. Grey seals in
the EEC performed their trips mostly in specific areas along the coast,
as the EEC is known to be a major ground for flatfish (Carpentier et
al., 2009; Riou et al., 2001; Selleslagh et al., 2009), which are
observed in grey seal diet (Planque pers. com) . In the ICS and
Iroise Sea, grey seals made shorter trips. These two sites are known as
highly biologically productive regions in the Eastern North Atlantic
(upwelling and area enclosed by specific currents, respectively for the
ICS and the Iroise Sea (Hily and Glémarec, 1999; Raine and McMahon,
1998). Furthermore, grey seals in these areas mainly feed on fish
species not targeted by commercial fisheries (Ridoux et al., 2007;
Vincent et al., 2016), and as a result, a low spatial overlap and low
resource competition were found between seals and fisheries (Cronin et
al., 2012). This low degree of competition with fisheries and high prey
availability could explain the shorter trips performed by these grey
seals, as they can find sufficient resource close to their haulout
sites. Furthermore, in the Iroise Sea, this was also supported by the
size of the colony (i.e. small local abundance at the southern
limit range), indicating a low effect of density dependence. Grey seals
in the Irish Sea adopted a different spatial usage compared to the four
other sites, with the highest trip duration. The numbers of seals
between the East and West parts of Ireland are of the same order of
magnitude (800 versus 1200 respectively), so colony size may not
explain the difference in spatial usage. However, a high degree of
interactions (i.e. spatial overlaps) between fisheries and grey
seals in the Irish Sea was recently highlighted (Cronin et al., 2016;
Gosch et al., 2019). These interactions with anthropic activities could
explain why grey seals made longer trips with an intermediate maximum
extent in this area compared to the other sites. But most of all, grey
seal selected foraging habitat matching those of their prey (Alheit and
Hagen, 1997; Gosch et al., 2014; Hammond et al., 1994; Ridoux et al.,
2007).