Alaska’s kelp forests are under threat from a combination of predator loss, ocean warming, and ecological chain reactions that have allowed sea urchin populations to explode. No single cause is responsible. Instead, several stressors have converged over the past few decades, and the damage has been most severe in the Aleutian Islands, where vast underwater forests have been reduced to barren rock.
The Sea Otter Collapse Started a Chain Reaction
Sea otters are one of the most important protectors of kelp forests. They eat sea urchins, the spiny grazers that devour kelp at the base and can strip an entire forest to nothing if left unchecked. When sea otter numbers are healthy, urchin populations stay small and kelp thrives. When otters disappear, urchins multiply and the kelp goes with them.
In the Aleutian Islands, sea otter populations dropped by roughly 75% between 1965 and 2000. At islands where otter numbers had been stable in 1965, the decline was even steeper: 88%. During the 1990s alone, the population shrank at an annual rate of about 17%. The leading hypothesis is that killer whales shifted to hunting sea otters after populations of their usual prey, seals and sea lions, collapsed. Without otters keeping urchins in check, the urchins consumed kelp forests across large stretches of the Aleutian chain, creating what biologists call “urchin barrens,” underwater deserts with almost no plant life.
A Mysterious Disease Wiped Out Another Urchin Predator
Sea otters aren’t the only animals that keep urchin numbers in balance. Sunflower sea stars, large predatory starfish that can span more than three feet across, are voracious urchin hunters. Starting in 2013, a devastating outbreak called sea star wasting syndrome swept the Pacific coast from Baja California to Alaska. An estimated 5.6 billion sunflower sea stars died, a 95% population loss. The species is now considered functionally extinct in California, Oregon, and coastal Washington, with only scattered individuals observed in recent years.
With sunflower stars gone, purple urchin populations boomed. The pattern played out dramatically along the Pacific coast: urchins that were once kept in check by both otters and sea stars suddenly had no significant predators in many areas. The resulting overgrazing has been one of the most destructive forces acting on kelp forests from Alaska southward.
Marine Heatwaves Are Cooking Kelp Directly
Even without urchin pressure, warming ocean temperatures threaten kelp on their own. Kelp species have specific temperature ranges they can tolerate. Sugar kelp, one of the most common species in Alaska, can survive water between about -1.5°C and 23°C, but growth and reproduction suffer well before those extremes. Bull kelp, another key species, shows stress in its spore development and settlement when temperatures rise beyond its optimum range. For both species, the early life stages (spores and microscopic reproductive cells) are especially vulnerable to warm water.
The most dramatic recent example was a massive marine heatwave nicknamed “The Blob” that persisted in the North Pacific from 2014 to 2016. In the Greater Farallones National Marine Sanctuary off California, the combination of the heatwave, the sea star die-off, and the resulting urchin boom led to a loss of 90% of bull kelp habitat. Alaska’s waters experienced the same heatwave, and while the impacts varied by region, the event demonstrated how quickly kelp forests can collapse when heat stress and predator loss overlap.
The Aleutians and Southeast Alaska Tell Different Stories
Not all of Alaska’s kelp forests are in the same condition. The differences between the Aleutian Islands and Southeast Alaska reveal how much local ecology matters.
In Southeast Alaska, when sea otters recolonized areas where they had been absent, the effects were dramatic. Urchin populations dropped by nearly 100%, and kelp rebounded quickly and significantly. The ecosystem snapped back. In the Aleutian Islands, the story has been far less encouraging. Even in areas where otters were present, urchin reductions were smaller (around 50%), and kelp recovery was much slower. The reason appears to be differences in how quickly new urchins are born and settle into the habitat. In the Aleutians, urchin recruitment is high enough that even with otter predation, new urchins keep arriving faster than otters can eat them. Otters also tend to target larger urchins, leaving smaller ones to continue grazing.
This means the Aleutian Islands face a structural disadvantage in kelp recovery. Even restoring otter populations there may not be enough on its own to bring forests back to their historical abundance.
Ocean Acidification Adds Another Layer of Stress
Alaska’s oceans are acidifying as they absorb more carbon dioxide from the atmosphere, and cold northern waters absorb CO2 more readily than warmer seas. This creates problems throughout the kelp forest food web. Research on pinto abalone, a common grazer found in Alaskan kelp forests, has shown that exposure to projected future winter acidity levels decreases both metabolism and growth. Coralline algae, which help stabilize the seafloor where kelp anchors itself, are also vulnerable to acidification because their calcium-based structures dissolve more easily in acidic water.
The concern isn’t just that individual species will struggle. It’s that acidification will reshape the relationships between species in ways that are hard to predict, potentially weakening the ecosystem’s ability to recover from other stressors.
What’s at Stake if Kelp Forests Disappear
Alaskan kelp forests support more than 20 species of fish, along with crustaceans, marine snails, sea urchins, starfish, and sea otters. Sea lions and whales use them as habitat. Every layer of a kelp forest, from the anchoring structures on the ocean floor to the dense mid-water stalks to the floating canopy at the surface, serves as food, shelter, or spawning ground for different species. Many commercially important fish depend on kelp forests as nursery habitat during their early life stages. Pacific herring, a keystone species in Alaska’s marine food web, is among those being studied for its vulnerability to changing kelp forest conditions.
The loss of kelp forests doesn’t just mean fewer plants underwater. It means the collapse of an entire habitat that supports fisheries, marine mammals, and coastal ecosystems across the state.
Early Restoration Efforts
Restoration work in Alaska is still in early stages but growing. The Alaska Fisheries Development Foundation has funded projects focused on developing hatchery methods for sugar kelp, including direct seeding techniques and large-scale bioreactor systems that could eventually produce kelp for both commercial farming and habitat restoration. Test outplantings are underway on farms in Juneau and Kodiak, where researchers are studying how different hatchery conditions affect kelp survival and growth once plants are placed in the ocean.
These projects are primarily geared toward kelp aquaculture rather than wild forest restoration, but the techniques being developed, particularly around growing kelp from spores and seeding it onto ropes or substrate, could eventually be applied to rebuilding wild populations. The bigger challenge remains addressing the root causes: restoring predator populations, managing urchin overpopulation, and slowing the ocean warming and acidification that make recovery harder with each passing year.