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Scottish Marine and Freshwater Science Reports

Formal report series, containing results of research and monitoring carried out by Marine Scotland Science


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Harbour seal diet composition and diversity

Scottish Marine and Freshwater Science Vol 7 No 21

The objective of this study was to provide seasonal and regional estimates of harbour seal diet composition and regional estimates of prey consumption, and to investigate how harbour seal diet (composition and/or diversity) relates to the different population trajectories observed around Scotland. In this report the work to address the following objectives is described: • Estimate harbour seal diet composition in 2010/11, regionally and seasonally; • Estimate the size of prey consumed by harbour seals, regionally and seasonally; • Examine if there is sex-specific variation in harbour seal diet; • Investigate how harbour seal diet relates to the different population trajectories observed around Scotland; • Estimate harbour seal prey consumption in 2010/11, regionally.

Wilson, L.J. & Hammond, P.S. (2016) Harbour seal diet composition and diversity. Scottish Marine and Freshwater Science Vol 7 No 21, 84pp. DOI: 10.7489/1801-1
Release Date
Spatial / Geographical Coverage Location
Temporal Coverage
2010-01-01 to 2011-12-31
UK Open Government Licence (OGL)
L J Wilson
Data Dictionary

Scats were collected in 2010/2011 up to two hours before and after low water (derived from POLTIPS, National Oceanographic Centre, NERC). Scats were placed into separate plastic bags and stored at -20°C. All scats collected were expected to be no more than two weeks old (since the last spring tide). Scat collections were stratified spatially and temporally and as weather allowed. Collections were distributed spatially in Scotland to match the Scottish Government designated Seal Management Regions (Baxter et al. 2011) and also included The Wash in south east England. All ten Special Areas of Conservation for harbour seals were included in the sampling programme. Table 1 lists all haul-out sites from which scats were successfully collected. Samples were collected in: summer (June–August, harbour seal pupping, breeding and early moult), autumn (September–November, harbour seal end of moult and start of the main period of foraging), winter (December–February, period mainly defined by foraging) and spring (March–May, pre-pupping), following Sharples et al. (2009). Table 2 shows the numbers of scats containing fish otoliths and or cephalopod beaks collected in each region/season and Table 3 shows the number of otoliths/beaks of each prey species recovered in scats, by region. Before approaching a haul-out site, the number of harbour seals was counted and any grey seals were identified and counted. Haul-out sites were designated as a single species site if the area contained =80% of one species (based on a low misclassification rate of 3% in molecular analyses to identify the species, Matejusová et al. 2013, Wilson 2014) or if the seals were spatially segregated at the haul-out site.

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Marine Scotland Science
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