Sympatric Seals, Satellite Tracking and Protected Areas: Habitat-Based Distribution Estimates for Conservation and Management

Abstract

Marine predator populations are crucial to the structure and functioning of ecosystems. Like many predator taxa, pinnipeds face an increasingly complex array of natural and anthropogenic threats. Understanding the relationship between at-sea processes and trends in abundance at land-based monitoring sites requires robust estimates of at-sea distribution, often on multi-region scales. Such an understanding is critical for effective conservation management, but estimates are often limited in spatial extent by spatial coverage of animal-borne tracking data. Grey (Halichoerus grypus) and harbour seals (Phoca vitulina) are sympatric predators in North Atlantic shelf seas. The United Kingdom (UK) and Ireland represents an important population centre for both species, and Special Areas of Conservation (SACs) are designated for their monitoring and protection. Here we use an extensive high-resolution GPS tracking dataset, unprecedented in both size (114 grey and 239 harbour seals) and spatial coverage, to model habitat preference and generate at-sea distribution estimates for the entire UK and Ireland populations of both species. We found regional differences in environmental drivers of distribution for both species which likely relate to regional variation in diet and population trends. Moreover, we provide SAC-specific estimates of at-sea distribution for use in marine spatial planning, demonstrating that hotspots of at-sea density in UK and Ireland-wide maps cannot always be apportioned to the nearest SAC. We show that for grey seals, colonial capital breeders, there is a mismatch between SACs (where impacts are likely to be detected) and areas where impacts are most likely to occur (at sea). We highlight an urgent need for further research to elucidate the links between at-sea distribution during the foraging season and population trends observed in SACs. More generally, we highlight that the potential for such a disconnect needs to be considered when designating and managing protected sites, particularly for species that aggregate to breed and exhibit partial migration (e.g. grey seals), or spatial variation in migration strategies. We demonstrate the use of strategic tracking efforts to predict distribution across multiple regions, but caution that such efforts should be mindful of the potential for differences in species-environment relationships despite similar accessible habitats.

Movements and spatial usage of harbour seals in the Elbe estuary in Germany

“…Along the North Sea coast of continental Europe and the UK, the harbour seal is an abundant predator and regarded as a key species in this ecosystem 3 , 14 . Previous studies have assessed the movement and habitat utilisation of harbour seals within the North Sea and adjacent coastal waters 13 , 15 , 53 , 54 . While there have been sightings of harbour seals in rivers and estuaries 19 , 20 , no studies have focused on how these seals utilise such brackish and fresh water environments adjacent to the North Sea.…”

Combining bioenergetics and movement models to improve understanding of the population consequences of disturbance

“… (A) At sea distribution (a proxy for animal density) of the two seal species on the east coast of Scotland (Carter et al 2020) and the 30 km radius areas of low and high densities are shown on the two left maps. The locations of the low‐ and high‐density areas for harbour porpoise in the Inner Danish water are shown in the right‐hand map.…”

“…We also used satellite tracking data from 12 adult HS and 19 adult GS that were tagged on the east coast of Scotland between 2008 and 2013 (see Carter et al 2020 for details on tagging and data processing). The duration of deployment was 10-160 days (mean 85 days) for HS and 14-230 days (mean 85 days) for GS For each simulated or tagged individual.…”

“…We carried out a systematic exploration of the effects of different values of these parameters. We then used outputs from individual-based movement models (Chudzinska et al 2021, Nabe-Nielsen et al 2014) and telemetry data (Carter et al 2022) to determine how the probability of exposure to these events might vary among individuals of the three species.…”

Insights obtained from a multidecadal citizen science scheme: grey seal (Halichoerus grypus) strandings in Cornwall and Isles of Scilly (2000–2020)

“…Following a post-weaning fast, during which they lose up to 25% of their body mass (Noren et al ., 2008), pups leave their natal site and move into an exploratory dispersal phase before settling, often a considerable distance from their place of birth (Carter et al ., 2020; Peschko et al ., 2020). Outside the breeding season, grey seals exhibit local redistributions and partial migration to foraging grounds – often several hundred kilometres offshore and are dispersed at much lower densities (Sayer et al ., 2021; Carter et al ., 2022).…”