Methods for estimating the population size of common seals, Phoca vitulina

In August 1985, an aerial survey was made over the coast of Orkney during the seals' moult. Results were compared with those from a series of boat surveys made over a sample of this area during the pupping season, in June and July. Over twice as many seals were found hauled-out on the survey made during the moult; future estimates of population size should be based on surveys made at this time of year. Time-lapse photography was used to look at changes in the number of common seals hauled-out in relation to the tidal cycle and the time of day. These data, together with data collected on the activity patterns of radio-tagged individuals, were used to provide correction factors to compensate for seal which were in the water at the time of the survey. Application of these correction factors to the survey total of 6616 produced a provisional estimate of 9331 (95% CLs 8147-10515) for the size of the Orkney common seal population. -from Authors