The short answer
Seals are almost exclusively fish-eating predators. An adult seal eats 3 to 7 kg of fish per day, depending on season, body weight and species. The Dutch Wadden and Delta waters provide a broad menu: flatfish (plaice, flounder, dab), cod-like species (cod, whiting, bib), herring, sprat and sand eel. Squid and shrimp make a smaller contribution.
The diet by prey type
- Flatfish (40–55%): plaice, flounder, dab and sole. The Waddenzee is extremely rich in these, especially in summer. Seals usually catch flatfish on the bottom.
- Cod-like species (15–25%): cod, whiting, bib and pollock. Grey seals especially eat relatively much cod, sometimes specimens up to a metre long.
- Herring and sprat (10–20%): caught in schools by both species, especially in the open parts of the Eems-Dollard and the North Sea coast.
- Sand eel (10–20%): a key prey. When sand eels vanish (water too warm), seal populations react immediately with leaner animals and higher pup mortality.
- Other (≤5%): shrimp, squid, crab, eel. Grey seals are opportunists and sometimes eat seabirds or young common-seal pups — exceptional, but documented.
How much per day?
An adult 90 kg common seal needs roughly 3 to 5 kg of fish per 24 hours. A 250 kg grey bull eats 5 to 7 kg, and more in colder months. That sounds like a lot, but in proportion a seal is efficient: about 5% of body weight per day. By comparison: a dolphin eats 4–6%, a human 2–3%.
During moulting (July–August for the common, February–March for the grey) seals eat far less; they live mostly off their blubber. The same applies to male grey seals in the mating season, who rarely leave the sandbar for four weeks and then lose 20% of their body weight.
Pup milk: extremely rich
A seal pup eats only mother's milk in the first 3 to 6 weeks of life. The milk is incredibly fatty: 45 to 60 percent fat content, compared to 4% in cow's milk and 8% in whale milk. It is one of the richest milks in the animal kingdom. As a result, a grey pup doubles its weight in three weeks — from 15 to 35–45 kg — and is ready to enter the water on its own. The mother loses around a quarter of her own body weight in that same time.
A weaned pup must then learn to hunt on its own. Many pups don't survive that transition; weakened animals wash up on beaches. Read our seal pup protocol if you find an emaciated young seal — never feed them yourself.
Difference between species
Both Dutch species eat broadly, but there are clear preferences:
| Species | Specialisation | Dive depth |
|---|---|---|
| Common seal | Flatfish, sand eel, smaller cod-like species | 10–50 m |
| Grey seal | Larger prey: adult cod, salmon, sometimes birds | 20–100 m |
The grey seal is "one size larger" in everything: bigger prey, deeper dives, longer foraging trips (up to 80 km from the resting site).
How they hunt: the whiskers
The secret of seal hunting lies not in the eyes, but in the whiskers (vibrissae). Each whisker is anchored in the skin by a dense bundle of nerve endings — thousands per hair. With them a seal can sense the tiny eddies left by a fish swimming away. In experiments, seals with covered eyes were still able to faithfully follow the path of a fish that had passed 30 seconds earlier.
This explains why seals are so successful in dark and turbid water: they barely need their eyes. Their hunting ground — the turbid Waddenzee — would be almost unusable for a fish species like tuna, but for a seal it is ideal.
For more on seal physiology, including the diving physiology that makes foraging possible, see anatomy and how deep do seals dive.
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