The Delta: Oosterschelde + Westerschelde
What we call "the Delta" in the Netherlands is in fact a collection of water and coastal systems in the south-west: the Haringvliet, the Hollandsch Diep, the Grevelingenmeer, the Veerse Meer, the Oosterschelde and the Westerschelde. For seals the last two matter most. Both are still openly connected to the North Sea — directly in the case of the Westerschelde, and semi-permeably in the case of the Oosterschelde since the storm-surge barrier was built.
At around 350 km² of water and roughly 110 km² of intertidal area exposed at low tide, the Oosterschelde is the larger of the two basins. The Westerschelde is slightly smaller in surface area but much longer (from Vlissingen to Antwerp) and has a stronger current. Both areas are Natura 2000 and form part of the international Birds and Habitats Directives.
How the Delta Works changed the habitat
After the 1953 North Sea Flood, the Netherlands began building the Delta Works: a series of dams and storm-surge barriers meant to protect the south-west of the country from a repeat. For seals, two structures were decisive:
- The Oosterscheldekering (1986). Instead of a full dam, the Netherlands chose a barrier that closes only when a storm surge is expected. As a result the Oosterschelde remained a salt, tidal, water-permeable area — unlike many other Delta basins that were sealed off completely.
- The mouth of the Westerschelde was kept open in the service of shipping to Antwerp.
The consequence is that intertidal zones — the strip that dries between high and low tide — have been preserved in both basins. That's crucial for seals. Without drying sandbars, no rest, no moult, no healthy population.
There is also a downside: the barrier means less sand is transported from the North Sea into the Oosterschelde. The deep channels and sandbars erode slowly — a process called "zandhonger" (sand hunger). Without intervention, drying sandbars eventually disappear. The project on the Roggenplaat in 2019 — adding 1.3 million m³ of sand — was a direct response to this.
The Oosterschelde as protected salt inland water
Because of the storm-surge barrier, the Oosterschelde is shielded from heavy North Sea swell in normal conditions, while the tide still flows through — at low water about a third of the area dries out. For seals this delivers a unique combination: a quiet, salt, tidal area that offers both rest and food.
The best-known haul-outs lie in the western and central section: the Roggenplaat, the Vondelingsplaat and the surrounding mud flats, and the bars along the Krabbenkreek near Sint-Annaland. In good conditions, hundreds of seals at a time are counted here at low tide.
The Westerschelde as a strong-flow estuary
The Westerschelde is a true estuary: a funnel-shaped river delta where fresh water from the Schelde meets salt water from the North Sea. The current is strong, the water turbid, and the salinity gradient runs from west (salt) to east (brackish). For seals it's a more challenging, more dynamic area than the Oosterschelde, but it does hold a growing common-seal population — especially on the mud flats at Saeftinghe, Hoofdplaat and in the mouth at Vlissingen.
The Westerschelde estuary is also important as a fish nursery. Young plaice, flounder, sole and sand eel grow up in its relatively warm, food-rich waters — direct prey for the seals on site.
Population growth since the 1990s
Before the Delta Works, the Delta was a heavily modified coastal system, and the seal population had never been as large as in the Wadden — and was very small for a long time afterwards. In the first years after 1986 the Oosterschelde seemed to be "slipping" in terms of seal recovery: the open connection to the North Sea had been reduced, the area was new to the species, and numbers stayed limited.
From the 1990s onwards — and more strongly after 2000 — numbers grew sharply. Two reasons are usually given:
- The hunting ban and Natura 2000 protection.
- The spontaneous establishment from the Belgian and Flemish coast, the Voordelta area, and — for the grey seal — from the United Kingdom.
The common seal is now a regular presence in the Oosterschelde, with several hundred animals at count moments. The grey seal followed later and has had a steady foothold in the Delta since the 2010s. Counts are carried out for Rijkswaterstaat.
Differences from the Wadden Sea
To anyone who knows both, the differences are immediately apparent:
- Smaller. The Delta as a whole is about a tenth of the Waddenzee. Fewer animals, but shorter distances between you and them.
- Different fish stock. The Westerschelde has its own estuarine fish community; the Oosterschelde behaves more like a salt inland sea with predominantly North Sea species.
- Easier to reach. Vlissingen, Goes and Zierikzee are directly accessible by public transport and car; for the Wadden you first have to catch a boat.
- Milder wind. No open North Sea expanse in front of the area; the wind builds less massively.
- Different demographics. As an area, the Delta population is younger than the Wadden population and is currently growing relatively faster.
Examples of regular haul-outs
A few of the haul-outs used year-round by seals:
- Roggenplaat (central Oosterschelde) — the Delta's main haul-out, raised in 2019.
- Vondelingsplaat — large sandbar south of Schouwen-Duiveland, in regular use.
- Mud flats near Sint-Annaland and Stavenisse — smaller haul-outs on the north side of the Oosterschelde.
- Mud flats near Saeftinghe — Westerschelde, on the Belgian border.
- Bars near Hoofdplaat and the mud flats near Breskens — Zeeuws-Vlaanderen side of the Westerschelde.
For specific shore spots: see the regional guide spotting in Zeeland. For the full filterable map: the spotting map. And to plan your visit well: read up on tides first.