Grey seal (Halichoerus grypus) pup production at North Rona: A study of birth and survival statistics collected in 1972

Authors

  • C. F. Summers,

    1. Seals Research Division, Institute for Marine Environmental Research, Natural Environment Research Council, c/o M.A.F.F., Fisheries Laboratory, Lowestoft, Suffolk
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  • R. W. Burton,

    1. Seals Research Division, Institute for Marine Environmental Research, Natural Environment Research Council, c/o M.A.F.F., Fisheries Laboratory, Lowestoft, Suffolk
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    • *Kirrin Lodge, Eredine, by Dalmally, Argyll.

  • Sheila S. Anderson

    1. Seals Research Division, Institute for Marine Environmental Research, Natural Environment Research Council, c/o M.A.F.F., Fisheries Laboratory, Lowestoft, Suffolk
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Abstract

The Grey seal breeding assembly on North Rona was studied throughout autumn 1972. Weekly censuses provided data on birth rate and mortality. It is estimated that 1736 pups were born in 1972. This is about 500 less than the mean production estimate for this assembly during the period 1959–71, possibly a result of disturbance of the breeding site by the expedition. About 600 of the pups died before putting to sea. The mean date of birth and the duration of the pupping season agree closely with earlier work on North Rona, although the mean daily birth rate curve is somewhat irregular, perhaps because there was an unusually dry spell of weather at a critical time in the pupping season. An improved estimate of mortality at this assembly showed a relationship with pup density similar to that observed at the Fame Islands. Techniques for estimating pup production (and consequently total population), though not reliable for determining absolute values, are useful for indicating trends and they suggest a stable population of 8000–9000 animals at North Rona. Such estimates might be improved by the use of better methods of aerial photography.

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