How Removing Sea Urchins Can Make Our southern California Kelp Beds Thriving Again

Updated: Oct 29, 2020

The species of kelp that make up California's kelp beds, Macrocystis pyrifera (Giant Kelp), can grow up to 40-65m high and at a speed of 60cm (2ft) a day! Animals depend on Giant Kelp beds for food and shelter. This rapid growth characteristic and use by other animals, makes Giant Kelp a foundation species. One of these species that relies on kelp as a food source is the purple sea urchin. Purple sea urchins in Southern California have always been present. But could a lack of predation on urchins and increased El Nino events make things worse for our kelp beds to recover?

Sea Urchins and Their Predators

Sea urchins, those prickly Echinoderms and cousins of seastars and sea cucumbers, are fed upon by their main predator: Southern Sea Otters. Unfortunately, sea otters once thrived from Baja California and around the Pacific Rim to Russia and Japan before fur hunters hunt them to near extinction in the 1700s and 1800s; the California population has grown from a group of about 50 survivors off Big Sur in 1938 to nearly 3,000 animals today (Monterey Bay Aquarium).

So what eats these purple sea urchins now? California sheepheads and California Spiny Lobsters are now the main predators of urchins in Southern California. However true, we have overfished these two predators, causing them to be of a smaller size and unable to eat the larger urchins, further causing a shift in these predators ecological role.

Sea urchins eat only the holdfast of Giant Kelp. A holdfast is a root-like structure of an algae; it is root-like in structure but not in function (it does not take in nutrients from the roots like in plants). For algae, holdfasts latch on to a substrate. Eating holdfasts can cause the whole kelp body to detach. Sea urchins can also lie dormant and wait for a new holdfast to start growing. One can start to see how this is a big problem... how is kelp supposed to recover if dormant urchins charge young kelp holdfasts?

Giant Kelp and The Science Behind Its Growth

This lack in sea urchin predation however, is not the only culprit preventing Giant Kelp from recovering: increased El Nino events are another factor. El Nino brings in warmer water to Southern California. Contrary to popular belief, warm water itself is not the issue for Giant Kelp, as Giant Kelp was found to grow in deeper tropical waters. Kelp, like all living organisms, has a limiting temperature that can prevent them from reproducing or eventually die if too hot. However, they can still thrive in warmer, tropical environments. In order to thrive, M. pyrifera needs the following for proper growth: 3 abiotic (non-living) factors -- a hard substrate to grow on, nutrients and plenty of light; and 2 biotic (living) factors -- herbivory (animals that eat kelp, like sea urchins) and competition for space (i.e. corals).

Kelp has been found in deep water habitats of tropical regions, up to 200m deep! In the tropics, the water is clearer and light can penetrate deeper in the water column, allowing light to reach the kelp at much deeper depths. So if warm water itself isn't the issue, than what is? Here's the science: warmer water tends to lack nutrients. If adequate nutrient concentrations are available in the water, kelps can survive up to temperatures near 23 degrees C (Santelices, B. 2007). In additions, kelp in tropical environments live lower down where the water temperature is going to be cooler (below 23 degrees C) where it can still thrive.

Competition wise, Giant Kelp cannot grow on coral. If corals are present, kelp cannot attach itself to the ground; remember, kelp needs a hard substrate, and therefore, cannot grow on sand and needs a rocky reef.

In summary, warm water itself is not necessarily preventing Giant Kelp from growing, but rather the lack of nutrients warm water carries. El Nino brings in oligotrophic (nutrient-poor) warmer water, making it difficult for Giant kelp to grow.

Hope for our Southern California Kelp Beds

With increasing El Nino events, in both frequency and intensity, scientists claim we will soon lose our kelp beds in SoCal. With El Nino AND having no chance to recover by the constantly eating sea urchins, how can we expect kelp to come back? Well, there is hope. Many scientists also believe that with anthropogenic aid, we can help prolong our kelp beds' lifespans. We believe we should act once a month, and act more during El Nino periods to minimize the impact the urchins have when El Nino passes. If we remove some urchins during El Nino, the kelp will recover much quicker once El Nino is over.

Project Kolika is in the works of launching a new campaign: The Giant Kelp Restoration Program, in hopes that by removing sea urchins and minimizing sea urchin barrens, we will allow a chance of recovery for our Giant Kelp. The logic is simple: remove a stressor (herbivorous urchins) of the Giant Kelp, therefore allowing Giant Kelp to recover, and thus, grow more rapidly.


1. Santelices, B. 2007. The discovery of kelp forests in deep-water habitats of tropical regions. Proceedings of the NationalAwan Riak Academy of Sciences 104: 19163-19164.

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