When Norwegian researcher Hans Kristian Strand boarded a research boat in the Arctic waters of northern Norway last May, he had low expectations. For years he had seen a gray, lifeless sea where tall kelp forests once waved beneath the waves, now replaced by hordes of sea urchins.
Strand and his colleagues had spent 25 years trying to bring back kelp and the balance it supports in a marine ecosystem, but hungry urchins repeatedly thwarted their efforts.
Efforts to tackle sea urchins
Kelp forests, found along about one-third of the world’s coastlines, are foundational to coastal oceans. They supply food and shelter for marine species, help purify water, sequester carbon and buffer shorelines from waves.
Over the past 40 years northern Norway — home to a large share of Europe’s marine forests — has lost at least 5,000 square kilometers of kelp. The decline began in the 1950s, when mechanization and a big increase in fishing reduced populations of predators like wolffish and haddock. Without those predators, sea urchins grazed kelp into disappearance.
Free diver Karoline Stigum Kvalo, who explores the Lofoten Islands, said the underwater life she now sees differs sharply from accounts from only a few decades ago. Divers recalled seeing so many wolffish in the 1990s that they were hard to avoid; now those fish are nearly gone.
Kvalo joined a free diver collective called the Kelp Watchers that tried restoring kelp by removing sea urchins, visiting sites once a month for more than a year, even in winter storms. Their work brought noticeable changes: many species returned when urchin numbers dropped. But the gains weren’t lasting. A sponge-like seaweed called lurv sometimes colonized cleared areas and blocked sunlight, preventing kelp from reestablishing.
A solution found on dry land
Kelp depends on a seasonal cycle: it blooms in spring and decomposes in autumn, creating spores that seed the next year’s growth. If kelp is absent in autumn, spores aren’t produced and nothing regrows in spring. That problem led Strand to an idea inspired by trees.
Walking in a pine forest, he noticed how conical trees capture sunlight and thought about creating a structure that could do something similar underwater. Collaborating with a local seaweed company, Polaralge, Strand’s team harvested sori — the reproductive tissue on mature kelp fronds — and placed them in tanks to release spores. They smeared the spore-rich water as a thin coating onto ropes, which were tied to sand-filled plastic rings to form suspended, climbable structures.
In February the team deployed these “artificial reefs” into the sea and exposed them to sunlight. Three months later, returning in spring, they found ropes cloaked in kelp that sea urchins could not reach. Earlier designs had failed, so seeing these structures abundantly colonized was a surprising and rare success.
To date they have placed 17 reefs along the northern Norwegian coast and plan to scale up to as many as 1,000 units made from black plastic piping filled with sand. The team has considered alternatives to plastic but has not found anything as easy to make and transport for broader use.
Why kelp forests matter
Kelp faces increasing pressure from warming seas, pollution and overfishing, making restoration urgent. Research by the climate nonprofit One Earth indicates kelp forests have declined by 40–60% over the past half century, with some areas such as parts of southern Australia and northern California losing as much as 95% of their giant and bull kelp.
Though largely out of sight, kelp matters to those who study and dive among it. Strand hopes the artificial reefs can offer a long-term way to revive kelp habitats; researchers from the Baltic region have already reached out. Meanwhile, the Kelp Watchers have paused their urchin-removal project. Kvalo says seeing kelp again has changed her diving: she now notices the tiny communities living on a single kelp leaf and the diversity it supports.
Edited by: Tamsin Walker