Photo @ DLNR Hawaii

Photo @ DLNR Hawaii

Sea urchins can have both positive and negative effects on coral reefs. In some ecosystems, they are key herbivores and play a critical role in maintaining the balance between coral and algae. In some situations, where urchin populations reach outbreak densities, they can lead to unsustainable bio-erosion.

Read more about effects of urchins on coral reefs and the possible reasons for sea urchin outbreaks.

Controlling Urchin Outbreaks

Urchin outbreaks are best managed by addressing the underlying causes, such as overfishing of predators or herbivores, or nutrient pollution. In some instances, however, rapid reductions in urchin density may be desirable to facilitate recovery of coral populations as part of a restoration strategy. Management trials, such as those in the Seychelles, have indicated that coral recruitment can increase up to two-fold at sites where urchins were removed. ref In Kenya, experiments also indicated that urchin removal can benefit corals, but that this can be preceded by an initial increase in seaweed abundance, and must also be accompanied by protection of fishes that prey on urchins. ref

Restoring urchin populations for herbivory

A recent effort to enhance recovery of the reefs in Kane‘ohe, Oahu, Hawai‘i is through herbivore enhancement. The area has been overgrown by invasive algae, including Gracilaria salicornia (also known as gorilla ogo) and the gristly yellow-green Eucheuma denticulatum, which smother the reef and destroy habitat for fish. In a healthy system, a native herbivorous fish and urchin community grazes the invasive algae. To restore health to the reef overgrown by algae, hatchery-raised juvenile collector urchins (Tripneustes gratilla) have been released onto the reef to graze the invasive algae and promote recovery of the reef.

Enhancing Urchin Populations

In some instances, managers are taking action to increase urchin populations to support reef recovery especially areas where populations of other herbivores (e.g., parrotfishes and rabbitfishes) have been depleted. For example, in the Caribbean, researchers are assessing the potential of growing sea urchin in labs and relocating them to the wild. In Hawaii, managers have raised and released collector urchins (Tripneustes gratilla) to graze the invasive algae and promote reef restoration.

Other actions have been explored to increase sea urchin populations, such as the building of artificial reefs which provide niches for urchins to hide from predators, and restrictions on fishing of urchin predators (e.g., triggerfish and larger wrasses) which could support the recovery of urchin populations. While a range of urchin enhancement projects have been attempted, the sustainability of these efforts remains in question. ref

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Last updated January 26, 2018

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