The number of pups born in a seal population can be used as an indicator of the size of the population. Each year, the Sea Mammals Research Unit (SMRU) conducts aerial surveys of the major grey seal breeding colonies in Britain to determine the number of pups born (pup production). These sites include about 85% of the number of pups born throughout Britain. The total number of seals associated with these regularly surveyed sites is estimated by applying a population model to the estimates of pup production. Estimates of the total number of seals at other breeding colonies that are surveyed less frequently are then added in to give an estimate of the total British grey seal population.
Each year, SMRU carries out surveys of common seals during the moult in August. It is impractical to survey the whole of the coastline every year but current plans by SMRU are to survey the whole coastline across 5 consecutive years. Seals spend the largest proportion of their time on land during the moult, at which time they are visible to be counted in the surveys. Most regions are surveyed by a method using thermographic, aerial photography to identify seals along the coastline. Conventional photography is used in the Wash. Additional surveys using visual counts are conducted annually in the Inner Moray Firth by the University of Aberdeen.
The estimated number of seals in a population based on most of these methods contains considerable uncertainties. The largest contribution to uncertainty is the proportion of the seals not counted during the survey because they are in the water. We cannot be certain what this proportion is and it is likely to vary from region to region and in relation to factors such as state of the tide and weather. Efforts are made to reduce the effect of these factors by standardising the weather conditions and always conducting surveys within 2 hours of low tide. About 40% of common seals are likely not to be counted during surveys but, because of the uncertainties involved in the surveys, figures are normally presented as minimum estimates of population size.