The status of grey seals in Britain

C D Duck,D Thompson

doi:10.7557/3.2723

Abstract

Grey seal pup production in Scotland was estimated through annual aerial surveys of the main grey seal breeding colonies. Between 3 and 7 counts of pups were obtained for each colony at intervals through the course of the breeding season. Pup production for individual colonies was estimated from the series of counts using a maximum likelihood model. At 3 colonies, 2 in England, annual pup production was estimated using ground counts. Between the early 1960s and the early 1990s, grey seal pup production progressively increased. At colonies in the Inner and Outer Hebrides, production appeared to stabilize during the 1990s and has remained so. Pup production at colonies in Orkney and in the North Sea has continued to increase but in recent years the rate of increase has declined. This may imply that the UK grey seal population is reaching some limit to its size. The observed changes in pup production imply that some density dependent factors are affecting the British grey seal population. Changes in either juvenile survival and/or female fecundity are the most likely options. Without knowing which of these, or what combination of these factors, is operating, estimating total population size is complicated.

Highlights

The population of grey seals in Britain, based on using pup counts as an index of abundance, has been increasing steadily since records began in the early 1960s (Summers 1978)
The total number of seals associated with these regularly surveyed sites is estimated by applying a population model to the estimates of pup production (Thomas and Harwood 2004)
Estimates of the total number of seals at other breeding colonies that are surveyed less frequently are added in to give an estimate of the total British grey seal population

Summary

Introduction

The population of grey seals in Britain, based on using pup counts as an index of abundance, has been increasing steadily since records began in the early 1960s (Summers 1978). The Sea Mammal Research Unit (SMRU) in St. Andrews, Scotland conducts aerial surveys of the major grey seal breeding colonies in Britain to determine the number of pups born (pup production; Duck 2004). Scotland conducts aerial surveys of the major grey seal breeding colonies in Britain to determine the number of pups born (pup production; Duck 2004) These colonies, most of which are in Scotland, account for about 85% of total British pup production. The total number of seals associated with these regularly surveyed sites is estimated by applying a population model to the estimates of pup production (Thomas and Harwood 2004). Estimates of the total number of seals at other breeding colonies that are surveyed less frequently are added in to give an estimate of the total British grey seal population

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