Shipworms destroy marina pilings in record time, costing Danish city €3.5 million

Wooden pilings in Vejle Marina, installed just 15 years ago, have been devoured by shipworms, forcing the city to replace them at a cost of 26 million Danish kroner (€3.5 million), DR reports.

A routine dive inspection in September 2024 revealed severe damage to the marina’s breakwater screen, with sections so weakened that “you could put your hand straight through it,” said Paw Bro Larsen, Vejle Municipality’s mobility chief. The discovery shocked officials, as the hardwood pilings—expected to last up to 50 years—had been reduced to a porous, crumbling state.

The infestation extends beyond the breakwater. Shipworms, a clam-like organism resembling earthworms, have also attacked the load-bearing pilings supporting the marina’s 200 berths. “This is a very robust wood type. To see it fail so quickly came as a complete surprise,” Larsen told DR.

Climate change accelerates decay
Researchers at Denmark’s National Museum, which has monitored shipworm activity since 1997, confirm the problem is worsening. Test blocks of wood submerged in Holbæk Harbor two years ago were “completely eaten,” said senior researcher Anne Marie Høier Eriksen. “The decay is happening twice as fast as we’ve seen historically.”

Rising sea temperatures have extended the shipworms’ breeding and feeding seasons, increasing their numbers and appetite. “Warmer water means a longer growth period and more individuals,” Eriksen explained. Even hardwoods—traditionally resistant to shipworms—are now vulnerable. “They’ll find wood to eat, no matter the type,” she warned.

Concrete replacement underway
Vejle Municipality and the marina will split the €3.5 million bill, with the city covering 20 million kroner. Work to replace wooden pilings with concrete began this spring and must finish by summer. “In a worst-case scenario, a severe storm could have collapsed the entire breakwater, sending waves directly into the marina,” Larsen said.

The National Museum’s tests show shipworms now target both soft pine and the hardwoods used in Vejle. “This is the same wood type they built with in Vejle,” Eriksen said, pointing to riddled test samples. “As you can see, it’s been attacked regardless.”