Methods for Estimating the Population Size of Common Seals, Phoca vitulina

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Abstract
This study evaluates the use of two different survey techniques for providing an estimate of the size of the common seal population in Orkney, U.K. In August 1985, an aerial survey was made over the coast of Orkney during the seals'' moult. These 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, and it is recommended that 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 seals which were in the water at the time of the survey. The application of these correction factors to the survey total of 6616 produced a provisional estimate of 9331 (95% C.L.s 8147-10515) for the size of the Orkney common seal population. This estimate is discussed in relation to previous estimates of the size of both the Orkney and the total British common seal population.