Scottish Marine and Freshwater Science Vol 8 No 25 | Updated Seal Usage Maps: The Estimated at-sea Distribution of Grey and Harbour Seals

In the previous UK-wide usage maps, count data between 1996 and 2013 were used (Jones et al., 2015). These count data were provided per haul-out site; the changes in the exact location of haul-out sites from year to year made it challenging to define effort historically. For count data between 1996 and 2015 (Figure 2), effort data (which is the same for both species) has now been defined on a (5 x 5 km) cell-by-cell basis (Figure 3). Thus for each year and cell, either a count (= 0) or a NA (not surveyed) was assigned. This has resulted in an increase in the accuracy of population estimates for each cell. Using these count data, the mean abundance of seals in 2015 has been estimated for each grid cell (see Jones et al. (2015) for detailed methods).

For a previous Scottish Government funded project (MMSS/001/11), seal telemetry data were combined with haul-out specific population data to generate usage maps for both grey and harbour seals around the UK at a spatial resolution of 5 x 5 km (Jones et al., 2013). These maps provided estimates of seal abundance (and associated confidence intervals) aggregated between 1988 and 2012, thus taking into account changes in population size through time. For Jones et al. (2015), maps were generated which were scaled to the estimated population size in 2013, resulting in the most up-to-date understanding of current seal usage at that time. The Department of Business and Industrial Strategy (BEIS) funded a large deployment of tags on grey seals in the southern North Sea and subsequently commissioned an updated North Sea usage map reflecting the estimated grey seal population size in 2015 (Jones and Russell, 2016). At the request of Scottish Government, usage maps have been generated for Orkney on a finer spatial resolution (0.6 x 0.6 km; Jones et al., 2016; Jones et al., 2017b).ChangesA requirement to update the UK-wide usage maps was identified. As part of this, three updates were proposed: (1) inclusion of additional telemetry data (up to 2016); (2) improvement in how count data are incorporated into the usage map framework (Jones et al., 2016; Jones et al., 2017a) and updating count data resulting in estimates of usage scaled to the estimated population size in 2015; and (3) clustering of haul-out sites to increase the proportion of sites for which there are associated telemetry data (Jones and Russell, 2016).