What does a Dutch seal eat?
An adult seal on the Dutch coast eats on average three to seven kilos of fish per day. Exactly how much depends on the species, the season and the body weight — a pregnant female or a moulting animal eats differently from a young subadult building its first reserves. On an annual basis it adds up to roughly one to one-and-a-half tonnes of fish per seal.
The menu is broad but predictable. Research on the faeces and stomach contents of Dutch and Wadden Sea seals shows the same groups again and again.
- FlatfishPlaice, flounder, dab, sole
- Cod relativesWhiting, cod, bib
- Pelagic fishHerring, sprat
- Sandy-bottom fishSandeel, smelt
- CrustaceansShrimp (mainly in young animals)
Young animals — pups in their first months on their own — often start with smaller prey: sandeel, gobies and shrimp. As they grow, they move on to bigger flatfish and cod relatives.
Difference between common and grey seal
The menu is largely the same, but the size of the prey differs. The grey seal is larger and so eats larger prey. In British and Dutch grey seals, remains of harbour porpoise and even seabirds are occasionally found in stomachs or faeces. In common seals that is rare to absent; they stick to fish.
| Aspect | Common seal | Grey seal |
|---|---|---|
| Average daily ration | 3–5 kg | 5–7 kg |
| Main diet | Flatfish, sandeel, herring | Larger flatfish, cod relatives |
| Outliers | Shrimp (young animals) | Sometimes seabirds, rarely porpoise |
| Hunting range | Mostly within 50 km of coast | Up to far offshore |
How do they hunt?
Seals hunt alone, not in groups like dolphins or orcas. There is no coordination, no rounding up of fish schools. What you see on a sandbar as social behaviour — lying next to each other, returning to the same site — does not apply under water. There, each seal is an individual hunter.
The hunt revolves around detecting prey in water where visibility is often barely two metres. A fish fleeing the seal leaves an invisible trail of eddies and pressure differences — a hydrodynamic "footprint" that lingers for seconds, sometimes minutes. The whiskers (vibrissae) on the snout pick up that trail. In German and Dutch experiments with blindfolded seals, the animals were able to follow a fish simulator for tens of seconds after it had passed, on vibrissae signal alone. More on that anatomical trickery on anatomy.
In turbid water the eye is almost useless — the seal doesn't see its fish, it feels where it just was.
Diving behaviour
Most meals are caught between ten and forty metres deep. On average, a Dutch seal dives to about 25 metres and stays down a few minutes. With larger prey or in deeper water, dives reach around 100 metres and outliers of twenty minutes occur. Before every dive, the following happens physiologically:
- The nostrils close actively — they are closed at rest and only open to draw breath.
- The ear openings seal under pressure.
- The lungs are emptied before the dive; oxygen is stored in blood and muscle tissue (myoglobin).
- The heart rate drops from about 120 beats per minute at the surface to under 10 at depth — the "dive reflex". Blood is shunted away from the limbs and skin and reserved for heart and brain.
Thanks to that physiology, a seal can dive repeatedly without becoming exhausted. A typical hunting sequence is a series of ten to fifteen dives, separated by short breathing pauses at the surface.
Energy needs through the year
Energy demand is not constant. It shifts with the cycle of moulting, gestation and nursing.
- Spring (mating)Males eat less, defend territory
- Summer (common seal nursing)Females fast or eat little
- Autumn (moulting)Much rest on the bar, lower intake
- Winter (grey seal nursing)Grey seal females fast for ~3 weeks
- Late winter (recovery)Intensive hunting to rebuild reserves
During the nursing period a seal mother produces remarkably rich milk: up to about 50% fat. The pup doubles in weight within three to four weeks on that diet. For the mother it costs a quarter of her body weight. Right after weaning, an intensive hunting period therefore begins to rebuild reserves — more on this on reproduction.
Food competition with fisheries
On the Dutch part of the North Sea and in the Waddenzee, humans and seals share the same food. Plaice, sole, cod and herring are economically important for the fishing industry and form the main diet of the seal. Studies show that the direct overlap is usually smaller than assumed — seals mostly catch smaller size classes than the fishery does — but local competition is real. Young seals also drown as bycatch, mainly in gillnets. On threats you'll find the figures and what is being done to reduce that bycatch.
The return of the grey seal (see species) has also changed the picture: a 200 kg animal eats more fish than a 90 kg common seal. As that population grows, so does the visibility of the debate — not always based on the most recent research data.
How do they see prey in turbid water?
The answer is that they largely don't. Visual hunting works mainly on clear days or in clear coastal water. In the Waddenzee, where turbidity from silt is often high, and on winter days without light, the vibrissae do the work. The seal swims in loops through an area, snout slightly forwards, until a trail is picked up. It then follows that trail — not the fish, but the eddies the fish has left behind — and closes its attack in the last few decimetres on a combination of sight and vibration. The whole body is built for this kind of hunting, from the streamlined shape to the blubber that lets it keep swimming in cold water. How that whole body fits together is set out on anatomy; how it plays out in everyday behaviour on the sandbar is there as well.