How We Can Help Keep Purple Urchins From Destroying California's Oceans

A towering forest of seaweed used to cover the bottom of the ocean along the Northern California coastline. These kelp forests would grow up to two feet a day, reaching heights of 30-60 feet tall. But in 2014, the warming of Northern California’s oceans became more intense, bringing with it an influx of purple urchins and spelling grave danger for California’s ocean life. 

Why is kelp so important?

The ecology of the entire stretch of California coast is dependent on a thriving kelp forest.  Unfortunately, since 2014 when the purple urchins first began to expand in population, 95% of the kelp across a large part of Northern California have disappeared.

The urchins have been particularly devastating for bull kelp, the main species of kelp in Northern California. The plants typically live about a year, returning again in the spring. But, the purple urchins are devouring the kelp spores as soon as the adult plants release them, keeping the plants from regenerating.

Where did the urchins come from?

Urchins are a normal part of the kelp forest, but two main factors have contributed to their recent overgrowth. 

Warming Ocean Waters: Studies show the hotter climate is making marine heatwaves more intense and more frequent. In 2014, A mass of warm water known as “The Blob”  began expanding. “The Blob” was low in nutrients, which made it more difficult for the kelp to grow.

Lack of predators:  The sunflower sea star is typically the purple sea urchins’ main predator on California's North Coast. The large sea star (also known as a starfish) has more than 20 arms and spans several feet across. But,  warmer ocean temperatures make sea stars more susceptible to diseases. Shortly before “The Blob” began expanding, sea star wasting disease spread across the West Coast, wiping out sea star populations. Sunflower sea stars are now listed as critically endangered on the IUCN Red List of Endangered Species.

How are the urchins able to survive? 

The urchins’ success is partly due to the elimination of most of the sunflower sea stars. Without their predators to keep them in check, urchins emerged from the crevices where they typically hid and became increasingly bold in competing for food.

"Sometimes we see dozens of them crawling up the stem of the kelp and taking it down from there," says Meredith McPherson, a graduate student at UC Santa Cruz.

Because the urchins require very little nutrition, they are able to thrive without eating for long stretches of time. 

How can we stop the urchins from taking over?

To give kelp forests a chance against the urchins, scientists, and divers are trying out new strategies to decrease the urchin population in small areas, in the hopes that the kelp will begin to make a comeback. They’ve developed three main strategies for removing the urchins.

Scoop up urchins

The decline in kelp also meant the disappearance of red urchins, which are more commercially valuable. To reduce the purple urchin population, commercial divers teamed up on a pilot project with Reef Check California, a group of citizen-scientist divers. The team found a small patch of kelp forest near Fort Bragg and began collecting as many urchins as they could. They delivered over 20,000 pounds of urchins to a composting facility. The project will continue this year in hopes that spores floating in from nearby healthy kelp forests will seed new growth.

Bring back predators

Since the hopes of bringing back sunflower sea stars any time soon are slim, scientists are looking into reintroducing other predators. California's sea otters, which were once near extinction, are beginning to increase in population in some parts of the state. While they’re not yet in Northern California, scientists have considered deliberately reintroducing the otters to Northern waters. 

Sea otters have a huge appetite and could make a big difference in reducing the purple urchin population. Unfortunately, while otters play a crucial role in maintaining healthy kelp forests, they generally steer clear of areas that are heavy on urchins because the urchins there are emaciated, and offer little nutritional value to otters. 

Eat them

In the past, most diners would eat the rich yellow reproductive organs of red urchins (called uni). Since the purple urchins are often emaciated and have little uni to offer, Stephanie Mutz, a diver who runs Sea Stephanie Fish, started an urchin ranching business. Mutz catches the purple urchins in the wild and fattens them up until they reach market weight.

Because so many Californians are aware of the devastating effect that purple urchins are having on their oceans, the demand has been high. Several other urchin ranching projects are also joining in to tackle the urchin problem. 

While none of these strategies are perfect, scientists are hopeful that through a combination of tactics, small pockets of kelp can begin to grow again, eventually leading to the thriving underwater forests that used to exist.