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Why Purple Urchins are Killing Kelp Forests

Why Purple Urchins are Killing Kelp Forests

Ocean forests are disappearing across the world. In California, we've lost up to 90% of our kelp forests in recent decades. There are different reasons as to why kelp is dying in each region, but we're going to deep dive into the story behind the disappearance of kelp just outside our door, in San Francisco.

Climate change almost eliminated our kelp forests. Over the last 100 years, the ocean warmed  1.5  degrees Fahrenheit. That might not sound like a lot, but it’s actually an enormous amount of heat; that kinda heat has enough power to transform marine biodiversity and change ocean chemistry. Scientists attribute the severity of sea star wasting syndrome (a disease that killed 5.75 billion and 90% sunflower sea stars) to warmer ocean temperatures. Sunflower sea stars are a top predator of purple sea urchins; so now we have no sunflower sea stars, and a ton of purple sea urchins.

Urchin barren

We're currently experiencing an explosion of purple sea urchin populations, and these urchins eat kelp. Purple urchins are actually native to kelp forests and play a role in balanced underwater ecosystems; however, with a surge in their populations, the kelp forests can’t keep up with their dietary demands. The urchins have munched up almost all of the kelp, leaving urchin barrens in their wake.

The urchin barrens look like dead, desolate ocean floors that lack any signs of life; except for the remaining “zombie urchins”, which are starving and barely alive without any kelp left to eat. Although giant kelp can grow up to 2ft per day, the zombie urchins suffocate the ocean floor and prohibit any growth. 

Urchin barren and oceanic dead zone

Our nonprofit partner SeaTrees is restoring balance to these ocean ecosystems through urchin culling; or removing urchins to allow the kelp forests to regrow. The urchins are removed by smashing them with a hammer. This might sound harsh, but it’s the most efficient way of restoring these underwater forests for 1,000+ other species that in the meantime, are lacking food & shelter.

When a zombie urchin is smashed, fish swarm to eat the leftovers. If kelp is still present in the area, simply culling the urchins will allow the kelp to grow back. If kelp isn’t present, then green gravel must be placed on the ocean floor. Green gravel is essentially kelp grown on rocks within aquaculture facilities; once the kelp has matured to a certain age, the rock can be thrown into the ocean and the kelp will regrow on its own.

Beautiful piece of kelp from kelp forest restoration

There are lots of way to kelp out! Every bag of kelp chips supports kelp restoration with SeaTrees. We’ve been able to restore 17,000 sqft of kelp forest with them so far! You can start kelpin' our oceans now by grabbing a bag of kelp chips here

If you'd like to learn more about kelp restoration projects, check out SeaTrees' website. If you'd like to get involved or volunteer, The Bay Foundation and Reef Check are great resources. Certified divers may have special opportunities with the right training!