More than 10 years after unusually high water temperatures in the Pacific Ocean killed off Northern California’s aquatic kelp forests, the region has still not recovered, reports David Helvarg, executive director of ocean conservation group Blue Frontier, for Mongabay.
From 2013 to 2017, a mass of unusually warm water nicknamed “the Blob” hugged California’s coast, leading to kelp forests collapsing. An estimated 95% of bull kelp (Nereocystis luetkeana), the main species of the region’s aquatic kelp forests, died.
Helvarg dove in Casper Cove, off Fort Bragg, a town in Northern California, and found that kelp forests there had still not recovered on a significant scale, except for some coves where more intensive recovery efforts are underway.
In one dive, he counted 120 purple urchins (Strongylocentrotus purpuratus) within what he guesstimated was just 1 square meter (11 urchins per square foot). Urchins devour weakened kelp forests, and for kelp to thrive, the number of urchins should be kept low, at around 2 per square meter, Helvarg notes.
Before the Blob, purple urchins were kept in check by the sunflower sea star (Pycnopodia helianthoide), a starfish that can grow up to 24 arms and weighs up to 5.9 kilograms (13 pounds). The critically endangered starfish is the major predator of urchins in Northern California, but the species lost 99% of its population to a disease outbreak that scientists say was helped by the warming waters.
Without sea stars, the purple sea urchins multiplied. One 2021 study estimated that their population grew by 10,000% in Northern California. The urchins, bolstered by the heat-weakened kelp, cleared around 563 kilometers (350 miles) of shoreline of kelp forests.
Getting rid of the purple urchins is hard work, as the species can survive in a dormant state for several years. This has left behind large stretches of rocky seafloor littered with urchins, which will reawaken when new kelp manages to take root.
In Casper Cove, dozens of divers have managed to clear more than 150,000 urchins from a 0.4 hectare (1-acre) stretch. But the task took three years, and hundreds of kilometers are still suffering from the impacts.
“There is nothing so beautiful as diving in a kelp forest, and I worry I may not get to do that again in my lifetime,” Sheila Semans, executive director of the Noyo Center for Marine Science in Fort Bragg, told Helvarg. “The kelp crisis will be a focus of attention for years to come.”
Read the full story by David Helvarg here.
Banner image: David Helvarg among the kelp stipes off Anacapa Island in southern California’s Channel Islands. Image courtesy of Jessie Altstatt.
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