e-Digest Statistics about: Wildlife
Wildlife monitoring
Seals
The grey seal is a relatively rare species and 40-45 per cent of the world's population breed in British waters. Britain is also home to nearly 40 per cent of the world population of the European sub-species of the common (or harbour) seal. The Sea Mammal Research Unit (SMRU) monitors changes in the size and status of stocks of grey seals and common seals in Britain through a programme of regular surveying and assessment. Estimates by SMRU [7], [8], [9] of the size of different seal stocks for the period 1986-98 are shown in Table 8. This table shows updated and revised population estimates based on improved modelling of variation in mortality and age of first breeding. The current table indicates that in recent years the common (or harbour) seal population in GB has begun to increase after a period of relative stability following the phocid distemper virus in 1988. The population of grey seals is continuing to rise and has more than doubled over the last 10 years.
The major grey seal breeding colonies have, until recently, been surveyed each year and smaller colonies every three to six years. The numbers of pups born in each colony are estimated using conventional aerial photography at the main breeding sites around Scotland. The methods used have been more or less consistent since 1984. The estimated GB totals in Table 8 for the all-age populations exclude Shetland and south west Britain which are not covered by regular SMRU surveys.
Surveys of common (or harbour) seal populations are concentrated at moulting time when more seals come ashore. Surveys in the past have been conducted annually in the Wash and biennially on the East Coast of Scotland (in the Tay and Moray Firths). In the future, surveys will be conducted every five years elsewhere in Scotland. Prior to 1988, counts were made from inflatable boats throughout the summer months. Since 1988, surveys have been conducted using a helicopter equipped with a thermal imaging camera; this allows a much greater area to be surveyed in a considerably reduced time. This change in survey method may account for some of the apparently large increases between recent counts and those from earlier boat surveys, for example the 1992 helicopter survey compared with the 1974 boat survey in the Outer Hebrides. The figures for common seals given in Table 8 represent estimates of the minimum number in each region.
In Detail:
MammalsFurther Information:
- Source:
- Sea Mammal Research Unit
Data Tables:
| Table
No. |
Title | Download
file type |
| 8 | Status of grey seals and common seals: 1986-1998 |
- References, further reading and links to other resources:
- [7] Harwood, J., Hiby, A.R., Thompson, D. and Ward, A., (1991). Seal Stocks in Great Britain: Surveys Conducted between 1986 and 1989. NERC News. NERC, Swindon.
- [8] Hiby, A.R., Duck, C. and Thompson, D., (1992). Seal Stocks in Great Britain: Surveys Conducted in 1990 and 1991. NERC News January. NERC, Swindon.
- [9] Hiby, A.R., Duck, C., Thompson, D., Hall, A., and Harwood, J., (1996). Seal Stocks in Great Britain. NERC News January 1996. NERC, Swindon.
- Internet Links:
- University of St Andrews: Sea Mammal Research Unit
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Page last modified: 16 September 2003
Page published: 10 September 2003