The biggest threats to the Dutch seal.

Bycatch in fishing nets

Bycatch — the unintended capture of a seal in fishing gear set for other targets — is one of the largest causes of death for seals in the Netherlands and the North Sea. The problem lies mainly with gill nets and trammel nets: passive nets that catch fish by snagging their gills. A seal swimming through in search of the same fish gets entangled and can no longer surface for air.

What makes this especially bad for seals: they have a relatively weak breathing reflex. Where cetaceans can subconsciously adjust their breathing, a seal switches less quickly into a panic response. Drowning sometimes occurs within minutes. Research by Wageningen Marine Research showed that bycatch in the North Sea affects hundreds to potentially over a thousand common and grey seals annually, especially in the southern North Sea and the coastal zone.

Plastic and net fragments

Entanglement in discarded fishing line, "ghost nets" (lost fishing gear) and plastic rings is a second major cause of injury and death. In adult animals these rings no longer grow out: a piece of net around the neck cuts through the skin, becomes infected, and kills the animal within months. In growing pups it happens faster.

The problem is largely inherited: a quarter or more of the marine plastic in the North Sea comes from fishing (lost nets, lines, buoys). The rest comes from shipping, recreation and rivers. Beach clean-ups help, but the scale is enormous. Wadden Sea beach clean-up actions and organisations like "The Ocean Cleanup" do what they can, but for seals the solution lies mainly at the source: better marking of fishing gear, biodegradable net materials and strict cleanup policy.

Human disturbance

A seal on a sandbar spends 30–40% of its time on dry land: resting, warming up, moulting and — for mothers — nursing. Every disturbance costs energy. A seal that is flushed 100 times enters 100 stress responses, loses 100 doses of nutrients to panic. For pups in the nursing period, a single disturbance can mean the difference between surviving and not.

The worst disturbers, in order of increasing severity:

• Walkers on salt marshes and sandbars in closed areas. Often well-meaning ("I only wanted to look"), structurally damaging.
• Kite surfers and sailboats passing too close to resting sites. The fast, unpredictable motion triggers a flight reflex.
• Dogs — loose, or leashed but too close. To a seal, dogs are predators.
• Drones — sound plus visual threat from the air (resembling an approaching bird of prey). Immediate panic response and flight.

Drones above seal habitat are forbidden within Natura 2000. Enforcement is difficult, but increasing. See protection for the legal framework.

Disease: PDV and avian flu

No threat is as visible as a disease outbreak. Three major epidemics have shaped the modern Dutch seal population:

PDV is a morbillivirus, related to canine distemper. It attacks the lungs and immune system; symptoms are coughing, nasal mucus, fever and swimming problems. Research points to migrating harp seals from the north as the original source. Whether a new PDV wave is coming is unpredictable — antibody studies show the Dutch population is partly immune, partly not.

Avian flu (H5N1) has struck seal populations worldwide since 2022. In the Netherlands, dozens of animals have tested positive, mostly adult grey seals. The disease is worrying because it suggests mammal-to-mammal transmission — a potential evolutionary step. Dead animals reported since then are routinely sampled by Wageningen Marine Research.

Food shifts and fisheries

Seals mainly eat flatfish (plaice, dab, sole), pelagic species like herring and sprat, and — especially the grey seal — cod-like fish. The North Sea is changing: climate warming is shifting species north, while fishing has thinned out certain stocks. The common seal, smaller and with relatively shorter foraging trips, seems more sensitive to local fish stock changes than the grey.

Researchers see this as one hypothesis for the stagnation and slight decline of the common seal in the Waddenzee since 2018 (see counts). Evidence is correlational, not definitive.

Bioaccumulation of pollutants

Seals sit high in the food chain and accumulate pollutants present in fish. Three groups matter:

• PCBs (polychlorinated biphenyls) — industrial substances, banned in the 1980s, but still present in sediment and fish. Affect reproductive success and immune system. Levels in Dutch seals have dropped significantly over the past 30 years, but are not gone.
• PFAS ("forever chemicals") — a rising concern since 2020. Detected in seal tissue; long-term effects still unclear.
• Mercury and other heavy metals — measurably elevated in older animals and grey seals; local fishing hotspots aggravate this.

Research by RIVM, Wageningen and Utrecht University tracks these substances annually. The message is twofold: the old pollutants are trending in the right direction, but the new ones (PFAS, microplastic additives) are not yet under control.

What can be improved?

Each major threat lends itself to concrete measures. The most achievable:

• Fishery adjustments. Time and area restrictions on gill nets in seal hotspots; pingers (acoustic deterrents) on nets; mandatory marking of fishing gear.
• Drone and kite-surf bans in Natura 2000. They already exist; enforcement must improve, especially in high season.
• Education for beach-goers. Dogs on the leash, 100 metres distance, don't intervene yourself — see seal pup found.
• Rapid disease monitoring. Report every dead animal, expand the necropsy protocol, develop vaccine research for PDV further.
• Source policy for pollutants. Stricter PFAS regulation, continued cleanup of old PCB hotspots in Rhine sediment.

None of these measures is a silver bullet. Together, however, they form the second generation of seal protection — after the 1962 hunting ban, the recovery decade of the 1980s, and the 2020 Seal Agreement.