Two weeks ago, President Donald Trump signed an executive order allowing commercial fishing inside nearly 1.3 million square kilometers of protected waters in the Pacific Ocean. The order, titled “Unleashing American Commercial Fishing in the Pacific,” stated that “appropriately managed commercial fishing” wouldn’t harm the marine reserve.
But recent research some 5,000 kilometers from this patch of the Pacific illustrates how even limited amounts of fishing can upend the effectiveness of a marine reserve in coping with ecological upheavals such as an underwater heatwave.
While these coastal ecosystems are vastly different from open ocean habitats, the findings off a broader lesson about how protecting species such as key predators can shape the fate of an ecosystem.
“This analysis provides robust evidence that protecting marine predators promotes the recovery of marine forests even in the face of prolonged marine heat waves,” said Stanford University marine scientist Fiorenza Micheli, an author of the paper that appeared in the journal Global Change Biology at the end of 2024.
Kelp forests are at the heart of the new work. Shallow coastal waters filled with the undulating “trunks” of golden-green kelp are havens for undersea life, sheltering fish, shrimp, crabs, and seals and sea otters, among others. But in California, these forests are disappearing, battered by hot water and an explosion in kelp-chomping sea urchins.
Following a record-setting marine heatwave dubbed “The Blob” that struck much of the West Coast between 2014 and 2016, coupled with a die-off of sunflower sea stars that prey on urchins, kelp forests in central and southern California shrank by roughly a third. In 2021, patches dotting the coastline totaled just 23 square kilometers, barely enough to cover half of Manhattan.
The destruction gave scientists an opportunity to see whether kelp forests inside state protected reserves fared any differently. While research has previously shown that marine reserves with strong protections can boost biodiversity, their effect on the ability to bounce back from a disaster have been less conclusive.
California’s reserves are unusually old and well-studied, offering the researchers a trove of data from which to try to answer these questions. They tapped into decades of satellite images to see how the size of kelp forests changed. Regular surveys of these forests and their inhabitants, along with similar counts in surrounding areas, enabled scientists to see how different species might be influencing the fate of these places.
In southern California, the analysis revealed that kelp forests inside reserves where fishing was completely banned declined less during the heatwave and recovered more than either unprotected forests or reserves with only partial limits on fishing.
The difference appears to trace back to the abundance of two species targeted by fishers – spiny lobsters and a fish known as a California sheephead. Lobster numbers were much higher in fully protected sites than in places with either partial or no protections, the scientists found. Sheepheads were more abundant in reserves with both partial and total fishing bans.
The urchin populations moved in the opposite direction of the lobsters and the sheephead, both of which dine on the urchins. That difference was strongest for fully protected sites, which experienced a statistically significant drop in urchins after the heatwave.
“Urchins eat kelp, so preserving their predators indirectly promotes kelp growth,” said Joy Kumagai, a Stanford biologist and Ph.D. student who was the paper’s lead author.
The role of this interaction between protected areas and predator-prey relationships was highlighted by the different results in central California. There, the main animals eating urchins are sea otters and the sea stars. But the marine reserves don’t affect where these creatures are found. The sea stars were decimated throughout the region by a disease in the 2010s. And sea otters are shielded everywhere by federal law. In that part of the state, kelp forests fared poorly in recent years regardless of marine protections.
In other words, the way a marine reserve influences the health of an ecosystem depends on the details. Even two ecosystems on the same coastline might go in different directions.
In the future, these kelp forests might be in even greater need of protections. In a paper published earlier this month in Nature Communications, some of the same scientists warned that by the end of the century, kelp around the world are likely to be hit by heatwaves with up to 16 times more intensity than they are now. And less than 3% of these forests have the kind of strict protections that benefitted southern California’s kelp.
Kumagai, et. al. “Marine Protected Areas That Preserve Trophic Cascades Promote Resilience of Kelp Forests to Marine Heatwaves.” Global Change Biology. Dec. 11, 2024.
Arafeh-Dalmau, et. al. “Global floating kelp forests have limited protection despite intensifying marine heatwave threats.” Nature Communications. April 3, 2025.
Image: ©Oleksandr Sushko via Unsplash
