The average lifespan
In the wild a seal lives on average 20 to 30 years. That applies to both Dutch species — the common and the grey seal — although the average for grey seals lies a little higher. The figures are based on counts of marked animals and on age estimates from growth rings in the teeth, comparable to tree rings in wood.
The spread around that average, however, is enormous. Many seals don't reach twenty, while exceptions can live well into their thirties or even forties. The oldest reliably documented grey seal reached 46 years, a female from Scotland; for the common seal the record is around 36 years.
Females older than males
A notable pattern: females systematically live longer than males, often by 5 to 10 years. The cause lies in the mating season. Grey males (bulls) fight each other for the favour of a harem and barely eat in that period; they can lose 20% of their body weight in four weeks. The deep scar stripes on the necks of older bulls are the visible price for that. In addition, young males die more often during dispersal to unknown habitats, where they have less experience with feeding grounds and currents.
Mortality by age class
The seal mortality curve is strongly age-dependent:
| Age | Annual mortality | Main causes |
|---|---|---|
| 0–1 year | 30–50% | Premature separation from mother, predation, undernourishment, parasites |
| 1–5 years | 10–15% | Fisheries bycatch, disturbance, lungworm infection |
| 5–20 years | 5–10% | Bycatch, disease (phocine distemper), shipping collisions |
| 20–30 years | 10–20% | Old age, tooth loss, hunting efficiency declines |
| 30+ years | 25%+ | Mainly old age |
The first twelve months are especially brutal: one in three to one in two pups doesn't reach its first birthday. Those that do survive the first year have a reasonably good chance of reaching twenty.
How ageing becomes visible
Experienced wardens recognise old seals fairly quickly from a number of features:
- Coat: thinner, duller and greyer. In old grey females the spotted pattern fades and the crown becomes nearly uniformly grey.
- Teeth: heavily worn or missing canines, especially in animals that ingest a lot of flatfish with sand. Tooth loss leads directly to lower hunting efficiency.
- Profile: leaner neck and flanks, especially at the end of winter. The blubber layer of an old animal recovers more slowly after a poor fishing season.
- Scar pattern: in old bulls the neck and shoulders are covered with parallel stripes from decades of fights.
- Movement: slower on land, shorter active periods in the water.
What determines the final end?
Most old seals die not from a specific disease, but from a cumulative combination: hunting efficiency drops → fat reserve drops → immune system weakens → parasites and lungworms take over. In the Netherlands, bycatch in gill nets is also a major human mortality cause, especially for males in spring. For population numbers and trends see our page counts; for anatomy and background seal anatomy.
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