Scottish Marine and Freshwater Science Vol 7 No 20
Since the last comprehensive assessment of grey seal diet around Britain in 2002, grey seal numbers have continued to rise in the North Sea while harbour seal numbers have declined in Shetland, Orkney and southeast Scotland. Stocks of gadid fish have also declined. In this report on task CSD3.3 of the MMSS/001/11 programme, grey seal diet is reassessed in 2010/11 and compared to previous assessments in 1985 and 2002, and estimates of prey consumed by grey seals are compared with fish stock sizes to estimate percent predation mortality. In this report on task CSD3 of the MMSS/001/11 programme, work to address the following objectives is described: • Estimate grey seal diet composition in 2010/11, regionally and seasonally; • Assess how diet around Britain in 2010/11 has changed compared to 1985 and 2002; • Estimate grey seal prey consumption in 2010/11, regionally; • Assess how consumption by seals as a percentage of estimated stock size of commercially important fish stocks in the North Sea (ICES Subarea IV) and west of Scotland (ICES Division VIa) has changed in 2010/11 compared to 1985 and 2002.
Data and Resources
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2010-01-01 to 2016-07-30
UK Open Government Licence (OGL)
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Diet composition and prey consumption were estimated using scat sampling methods as previously used in 1985 and 2002 and as described in detail in previous reports to Defra and Scottish Government (Hammond and Grellier 2006, Hammond and Harris 2006). Scats were collected on a quarterly basis for one year in 2010/11 around Scotland and along the east coast of England. Fish otoliths and cephalopod beaks recovered from scats were identified and measured, corrected for partial and complete digestion using experimentally derived coefficients, and the data used to estimate the contribution of each prey species to the diet as a proportion of the total estimated weight consumed. Diet composition was estimated regionally (Inner Hebrides, Outer Hebrides, Shetland, Orkney and northern North Sea, central North Sea and southern North Sea) and seasonally. Diet composition results were used to estimate the amount consumed of each prey species, assuming that grey seals, on average, met their estimated energy requirements. Annual consumption estimates were compared with the estimated size of stocks assessed by ICES in Subarea IV (North Sea) and Division VIa (west of Scotland). Results for 2010/11 were compared with those previously presented for 1985 and 2002.
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