The short answer
Harbour (common) seals in the Netherlands moult in summer, mainly in July and August. Grey seals moult in spring, roughly February to April. Moulting means the old hair falls out and a new coat grows.
What happens during the moult?
A year of salt water, sun and sand wears a seal’s coat down, so it is replaced once a year. In harbour and grey seals this happens gradually: old hair falls out in patches, often first around the head and flippers, so a moulting seal can look blotchy or bald. That is normal and not a sign of disease.
New hair needs warm, well-perfused skin. In cold water that would cost a lot of heat, so seals spend much longer hauled out on sandbars during the moult. They also eat less and live off their blubber.
Timing and duration
For one seal the moult lasts a few weeks. Not all animals start together — young animals often moult earlier than adult females, which first nurse their pups — so the moult of a population stretches over two to three months.
| Harbour (common) seal | Grey seal | |
|---|---|---|
| Moult (NL) | mainly July – August | mainly February – April |
| Before that | pupping and nursing (June – July) | pupping and nursing (December – January) |
| Moult count | August | March – April |
Why researchers count during the moult
Because a large share of the population is on land at the same time, the moult is the best moment to count. Survey flights by Wageningen Marine Research and colleagues in Germany and Denmark count harbour seals in August and grey seals in spring. See the results on seal counts.
Pups: the first coat
Grey seal pups are born with a white, woolly coat (lanugo) that they shed at about three weeks, around the end of nursing; only then do they go to sea. Harbour seal pups usually shed their lanugo before birth and can swim straight away.
Give moulting seals space
A seal repeatedly chased into the water during its moult loses a lot of energy — a real risk in summer when beaches and the Wadden Sea are busy. Keep well away from haul-outs, also by boat or kayak, and keep dogs on a lead. A bald-looking seal on the beach is not necessarily ill; if in doubt, in the Netherlands call 144. More on the yearly cycle on life cycle and on timing your visit on when to spot seals.
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