How are the counts done?
Seal counts in the Netherlands have been running since the 1960s. Since the 1990s they have been standardised, with the work carried out by Wageningen Marine Research (formerly IMARES) on behalf of the Ministry of LNV. The method: aerial counts during specific months, on fixed routes, at low tide when seals lie on sandbars and salt marshes.
For the common seal counts take place in the summer months (June–August), the period of birth and moulting when animals lie out longer and more visibly. For the grey seal, there are two counts a year: one in winter (November–January) during the pupping season, and one in spring (March–April) during the moult.
The figures are minimum estimates: only animals lying out are counted. Swimming or foraging animals are missed. Statisticians correct with "haul-out factors" — estimated percentages of the population that is on dry land at a given moment. The true population is therefore larger than the raw count.
Dutch counts are coordinated with those in Germany (Schleswig-Holstein, Lower Saxony) and Denmark via the Trilateral Wadden Sea Secretariat (TWSS). Common seals are counted in coordinated flights in June (pups) and August (moult); grey seals in winter (pups) and spring (moult). Teams fly as close to simultaneously as possible to avoid double-counting.
Common seal — Dutch Waddenzee
After decades of hunting and pollution, only 480 common seals were counted in the Dutch Wadden area in 1976, and just over ten in the Delta. After the hunting ban (1962) and the reduction of PCBs the population recovered, despite two outbreaks of phocine distemper virus (PDV) in 1988 and 2002 that each killed roughly half of the animals. The peak came in 2013: 7,605 common seals in the Dutch Wadden area and 597 in the Delta.
Growth then stalled and numbers fell from 2022, without any observed mass mortality. In the August 2025 moult count, 7,285 common seals were counted in the Dutch Wadden Sea, 10% more than in 2024. The number of pups in the Netherlands rose 44% to 2,809 in 2025.
| Year | Common seal, Dutch Wadden area (moult count) | Note |
|---|---|---|
| 1976 | 480 | low point |
| 1988 and 2002 | — | PDV epidemics, each ~50% mortality |
| 2013 | 7,605 | highest count |
| 2022–2024 | — | declining numbers |
| 2025 | 7,285 | +10% vs 2024; 2,809 pups |
Sources: Compendium voor de Leefomgeving (common and grey seal indicator 1960–2025) and Wageningen Marine Research / Expert Group Marine Mammals (2025 survey).
Grey seal — Dutch Waddenzee
The grey seal is the great success story of Dutch marine mammals. After centuries of absence it has been back since 1980, and the first pup was born in 1985. Growth is driven mainly by immigration from the British Isles. De Richel between Vlieland and Terschelling is the best-known breeding site.
| Count | Whole Wadden Sea + Helgoland | Of which Netherlands |
|---|---|---|
| Moult, spring 2025 | 12,064 | 8,638 |
| Moult, spring 2026 | 12,497 (+3.6%) | 8,981 |
| Pups, winter 2025–2026 | 3,385 (+10%) | 1,731 |
Pup numbers grew by an average of about 12% a year over the past five years, while growth in the moult counts of adults is levelling off.
Delta — Voordelta, Oosterschelde, Westerschelde
Besides the Wadden Sea, the Delta is the second seal habitat in the Netherlands. According to the Compendium voor de Leefomgeving, 1,738 common seals were counted in the Delta in August 2023 — a huge increase from the handful of animals in the 1970s. Delta counts of grey seals fluctuate strongly: 3,467 in 2023 (an exceptional peak) and 2,901 in 2024. Many grey seals move between the Delta, the Wadden Sea and British colonies. For local details see Delta.
Trends and hypotheses for the common seal stagnation
Why did the common seal stall after 2013 and decline from 2022, while the grey seal is still growing? Researchers are working on several non-exclusive hypotheses:
- Food competition with the grey seal. The grey is bigger, eats many cod-like fish, and can swim in the same places. Strong growth of the grey population can squeeze food availability for the smaller common seal.
- Predation by grey seals. Grey seals are known to sometimes attack and kill other seals (and harbour porpoises). How much this affects the population is unknown.
- Local disease incidents, including avian flu (H5N1) since 2022.
- Changing food availability due to climate and fisheries.
- Carrying capacity reached. The common seal may sit at or near the natural maximum population for the Dutch Waddenzee.
The cause is not yet known; Wageningen Marine Research and partners are investigating. Follow-up research is ongoing; read more about the causes on threats.
Trilateral Wadden numbers
Seals cross borders. For the full picture, the trilateral figures (Netherlands, Germany, Denmark and Helgoland) matter more than the Dutch part alone:
| Species and count | Total Wadden Sea + Helgoland | Netherlands |
|---|---|---|
| Common seal, moult Aug 2025 | 23,954 (+1%) | 7,285 |
| Common seal, pups June 2025 | 10,044 (+22%) | 2,809 |
| Grey seal, moult spring 2026 | 12,497 | 8,981 |
| Grey seal, pups winter 2025–2026 | 3,385 | 1,731 |
The total of 23,954 common seals is still below every count from 2012–2021; since 2021 the moult counts show a declining trend of about 4% a year. Pup numbers in 2025, by contrast, were the second highest ever recorded. Sources: "Survey results of Harbour Seals in the Wadden Sea in 2025" and "Grey Seal Surveys of the Wadden Sea and Helgoland 2025–2026" (Trilateral Wadden Sea Cooperation).
Sources
The official, annually updated sources for Dutch and trilateral seal numbers are:
- Wageningen Marine Research — annual Dutch count reports for common and grey seal, on behalf of the Ministry of LNV.
- Trilateral Wadden Sea Secretariat (TWSS / CWSS) — joint annual reports "Wadden Sea Harbour Seals" and "Wadden Sea Grey Seals" with figures per sub-region.
- Compendium voor de Leefomgeving (CLO) — Dutch nature indicators with graphs and long time series.
- Statistics Netherlands (CBS) — aggregated data on marine mammals in Dutch waters.
We update this page annually when the new report from Wageningen Marine Research appears — usually in autumn. Figures on this page are indicative; for scientific use, always consult the original.