moult count

A survey of seals made during the moult, the weeks each year when they come ashore to shed and regrow their coat, used to track how a population changes.

Once a year seals moult, shedding and regrowing their coat, and during those weeks they haul out, or come ashore, on undisturbed sandbanks to rest. A moult count is a survey timed to that window. In the Wadden Sea, the stretch of coast shared by the Netherlands, Germany and Denmark, it is flown from aircraft on dates agreed between the three countries, so the whole coast is counted the same way.

Timing differs by species. In the Wadden Sea the moult of adult grey seals peaks between March and April, while harbour seals moult mainly in August. The trilateral Seal Management Plan sets a minimum of five aerial surveys a year for each species: two during the moulting period and three during the pupping season, when the young are counted.

A moult count records the seals ashore on the survey days, not every seal in a population. Seals move between colonies, and many that moult in one place breed somewhere else, so a single year's figure can rise or fall with weather, disturbance and visitors. Its value lies in repetition: counted the same way every year, it builds a long-term record of how a population's size and distribution are changing.

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