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–February) during the nursing season, and one in spring during moulting.
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 may therefore be 20–40% higher 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). On one day in June and one day in November, teams fly simultaneously over their stretch of the Waddenzee, so that double-counting is avoided.
Common seal — Dutch Waddenzee
The Dutch Waddenzee common seal population recovered steadily from the 1962 hunting ban, in particular after the PDV setbacks of 1988 and 2002. Between 2013 and 2018 the population reached a peak of around 7,000 animals. Since then growth has stagnated and recent years have shown a slight decline. The 2024 count reports approximately 6,000 common seals in the Dutch Waddenzee.
| Year (count) | Common seal NL Waddenzee | Trend |
|---|---|---|
| ~1960 | a few hundred | low point |
| 1988 | ~2,500 (post outbreak) | PDV blow |
| 2002 | ~3,500 (post outbreak) | PDV blow |
| 2013 | ~7,000 | peak reached |
| 2018 | ~7,000 | stagnation begins |
| 2024 | ~6,000 | slight decline |
Note: exact figures differ by source and counting moment. Wageningen Marine Research publishes annual reports with the most precise numbers; always use the most recent annual report for citation.
Grey seal — Dutch Waddenzee
The grey seal is the success story of Dutch marine mammals. Virtually absent in 1980, first pup in 1985, and in 2024 around 7,800 animals in the Dutch Waddenzee. De Richel between Vlieland and Terschelling is the main nursery; groups also live on Rottumerplaat, Engelsmanplaat and Schiermonnikoog-north.
| Year | Grey seal NL Waddenzee | Milestone |
|---|---|---|
| 1980 | a few dozen wanderers | return begins |
| 1985 | — | first pup in centuries |
| 2000 | ~600 | stable colony |
| 2015 | ~4,000 | accelerated growth |
| 2020 | ~6,500 | — |
| 2024 | ~7,800 | continued growth |
Growth appears to be levelling off in recent years; ecologists expect the Dutch Waddenzee may be approaching its carrying capacity for the grey seal. Definitive statements require more years of data.
Delta — Voordelta, Oosterschelde, Westerschelde
Alongside the Waddenzee, the Delta is the second seal habitat of the Netherlands. Here both species are on the rise:
- Common seal Delta: approximately 1,500 animals in 2024, mainly in the Voordelta and on the Hooge Platen.
- Grey seal Delta: a few hundred animals, with a growing nursery on the Roggenplaat and in the Voordelta. The first active breeding was confirmed around 2014–2015.
Delta counts are also carried out by Wageningen Marine Research. For local detail see Delta.
Trends and hypotheses for the common seal stagnation
Why is the common seal stagnating and declining since 2018, 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 on common-seal pups. Since 2010, confirmed cases exist in which grey seals attack and kill common-seal pups. The scale is contested, possibly significant.
- 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 Trilateral Wadden Sea Conference of 2022 noted the stagnation as an explicit area of attention. Follow-up research is ongoing; read more about the causes on threats.
Trilateral Wadden numbers
Seals cross national borders. For the full picture, the trilateral figures (NL + Germany + Denmark) are more relevant than the Dutch share alone. The 2024 figures (rounded, indicative):
| Species | NL | DE (Schleswig-Holstein + Lower Saxony) | DK | Wadden Sea total |
|---|---|---|---|---|
| Common seal | ~6,000 | ~17,000 | ~3,500 | ~26,500 |
| Grey seal | ~7,800 | ~3,000–4,000 | a few hundred | ~11,500 |
The above figures are rounded indications; consult the most recent TWSS report ("Wadden Sea Seal Report") for exact numbers.
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.