Seals and humans are top predators in many marine ecosystems, often targeting the same food resource. With global declines in fish stocks, competition between these top predators is of increasing interest to scientists and resource managers. To understand and quantify this competition, robust data on the diet of seals are necessary. We present new information on the diet of grey seals Halichoerus grypus from a colony on Great Blasket Island, southwest Ireland, with particular reference to commercially important prey species such as salmonids. Inter-annual variation in diet was investigated based on the recovery and identification of sagittal fish otoliths, bones and cephalopod beaks from faecal samples collected in 2009 and 2010. A total of 939 prey items were recovered, representing a minimum of 41 prey species. Thirty species were identified from otoliths while a further 11 species, including salmonids, were identified from additional diagnostic structures. The diet of grey seals was largely dominated by Gadiformes (21% corrected biomass), particularly Trisopterus spp., which contributed 7% in terms of corrected biomass and 52% by frequency of occurrence, while salmonids comprised over 36% of diet by corrected biomass. Inter-annual variation in sandeel (Ammodytidae) and blue whiting Micromesistius poutassou abundance was apparent, while the former was an important contributor to the diet in terms of corrected biomass (11%). The results support the principle that an 'all structures' approach to pinniped diet estimation can significantly increase the rate of detection for most fish groups, and result in improved estimation of seal diet.

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1.We constructed a size-and trait-based dynamic marine community model of the Celtic Sea/Biologically Sensitive Area, including grey seals Halichoerus grypus (Fabricius 1791) and harbour seals Phoca vitulina vitulina (Linnaeus 1758) to examine potential resource conflict between seals and commercial trawl fisheries. The model incorporates seal diet preference, population size and commercial fishery catch, with survey data to quantify ecological interactions between seals and fisheries. 2. Total annual consumption by seals was an order of magnitude less than the catch of the modelled trawl fishery. Increasing fishing pressure reduced fish spawning stock biomass (SSB) much more than a proportionally equivalent increase in seal predation. For most fish species, quadrupling seal predation showed little effect on the predicted fishery catch. 3. These results arise from relatively low seal abundance and partial niche partitioning. The fishery harvested a wider range of fish lengths and species than seals consumed. The fish community was dominated by small fish lengths and seals predated on these more than suggested by their calculated diet preference. 4. Seal predation disproportionately affected several fish species not targeted by the fishery, but seal predation did not significantly affect the SSB of any of the species that constitute 90% of the total landings of the fishery. 5. Synthesis and applications. Predation of fish by grey and harbour seals is unlikely to harm commercial trawl fisheries in south-west Irish waters. This conclusion differs from those of some model-based studies of other North Atlantic systems, demonstrating the need for ecosystem-specific evidence in considering such conflicts. In systems with low niche overlap between seals and fisheries, the two are largely decoupled in effect, leaving fishing pressure as the overwhelming determinant of targeted fish stock status.

In ecological studies it is often assumed that predator foraging strategies and resource use are geographically and seasonally homogeneous, resulting in relatively static trophic relationships. However, certain centrally placed foragers (e.g. seals) often have terrestrial sites for breeding, resting, and moulting that are geographically distinct, and associated with different habitat types. Therefore, accurate estimations of predator diet at relevant spatial and temporal scales are key to understanding energetic requirements, predator-prey interactions and ecosystem structure. We investigate geographic variation in the diet of grey seals (Halichoerus grypus), a relatively abundant and widely distributed central place forager, to provide insights into geographic variation in resource use. Prey composition was identified using scat samples collected over concurrent timescales and a multivariate approach was used to analyse diet from two contrasting habitats. Regional differences in prey assemblages occurred within all years (2011–2013) and all seasons (ANOSIM, all p<0.05), apart from in winter. Telemetry data were used to identify core foraging areas and habitats most likely associated with scat samples collected at the two haul-out sites. Regional differences in the diet appear to reflect regional differences in the physical habitat features, with seals foraging in deeper waters over sandy substrates showing a higher prevalence of pelagic and bentho-pelagic prey species such as blue whiting and sandeels. Conversely, seals foraging in comparatively shallow waters had a greater contribution of demersal and groundfish species such as cephalopods and flatfish in their diet. We suggest that shallower waters enable seals to spend more time foraging along the benthos while remaining within aerobic dive limits, resulting in more benthic species in the diet. In contrast, the diet of seals hauled-out in areas adjacent to deeper waters indicates that either seals engage in a more pelagic foraging strategy, or that seals can spend less time at the benthos, resulting in comparatively more pelagic prey recovered in the diet. The substantial differences in prey assemblages over a small spatial scale (<300 km) demonstrates the importance of using regionally-specific diet information in ecosystem-based models to better account for different trophic interactions.