Coral reefs can’t survive rising sea urchin numbers

Large populations of spiny sea urchins are invading Hawaii’s coral reefs, placing these essential underwater ecosystems at severe risk.

Recent studies show that reefs may be eroding faster than they can naturally rebuild due to a growing imbalance in marine life.

These concerns stem primarily from warming ocean temperatures, water pollution from land-based sources, and ballooning numbers of sea urchins.

Researchers have linked the rapid rise in urchin populations to a steep decline in certain fish species, mainly those targeted by fishing, that usually regulate these grazers.

How sea urchins damage coral reefs

Sea urchins feed heavily on coral reefs, gradually scraping away at the coral’s protective limestone skeleton. While these creatures naturally coexist with reefs, an excessively high population can severely weaken reef structures through bioerosion.

“We are seeing areas where you have about 51 urchins per square meter, which is among the highest population density for sea urchins anywhere in the world,” said Kelly van Woesik, a Ph.D. student in the North Carolina State University Center for Geospatial Analytics.

Major reef decline in Hawaii

Hōnaunau Bay, located on Hawaii’s western coast, is particularly affected. Known historically for vibrant coral reefs and clear waters, researchers now report significantly reduced coral growth.

Recent surveys show that the reef’s net carbonate production, a critical measure of coral growth, is alarmingly low. Areas previously flourishing now face difficulty maintaining the minimal coral coverage required to balance natural erosion, particularly in shallow regions.

Coral coverage in Hōnaunau Bay currently averages around 28%, just above the critical 26% threshold needed for positive growth. However, certain shallower areas require nearly 40% coral cover to offset severe erosion pressures from high urchin densities.

Pollution and climate stress

Water pollution from agricultural runoff and urban sources introduces excess nutrients and chemicals, disrupting water chemistry. Changes in nutrient levels and acidity from pollution directly harm coral, making reefs less resilient against natural stressors.

Climate change exacerbates this issue further. Rising ocean temperatures cause coral bleaching, weakening coral defenses against diseases and pests. Each bleaching event reduces coral growth potential and increases vulnerability to urchin-driven erosion.

Reef collapse and urchin surge

The link between overfishing and reef health is increasingly clear. With fewer predatory fish, such as triggerfish and wrasses, urchins face reduced natural controls, allowing their populations to surge dramatically.

This disruption extends beyond predatory fish. Herbivorous fish like parrotfish are also declining, reducing algae control. Unchecked algae growth further competes with corals for space and resources, diminishing reef resilience and biodiversity.

Studies from other regions show that areas protected from fishing typically experience healthier fish populations, lower urchin numbers, and more robust coral reef ecosystems. Hawaii’s current situation starkly contrasts these healthier ecosystems, highlighting urgent conservation needs.

Impacts on coastal communities

The declining health of reefs impacts coastal communities profoundly. Coral reefs serve as natural breakwaters, absorbing up to 97% of wave energy, significantly protecting shorelines against storms and coastal erosion.

With reefs compromised, coastal properties and infrastructure face increased vulnerability, especially in areas dependent on tourism. In Hawaii, reef-based tourism, snorkeling, diving, recreational fishing, generates substantial economic activity and employment.

Coral reef deterioration directly threatens these economic benefits. Healthy reefs attract tourists; degraded reefs could thus lead to substantial economic and social consequences.

Broader effects of urchin-damaged reefs

Coral reefs hold profound cultural significance for Native Hawaiians, who have historically relied on reefs for food, medicine, and spiritual practices.

Traditional management systems once preserved the natural balance of these ecosystems, reflecting deep knowledge and respect for marine resources.

As reef health deteriorates, cultural traditions connected to the reefs risk fading away. Restoring reefs therefore means more than environmental or economic recovery; it also involves reclaiming cultural heritage and revitalizing traditional stewardship practices.

Addressing the crisis

Scientists stress immediate action to restore marine balance. Better fisheries management emerges as a critical step, as replenishing predator populations can help naturally regulate urchin numbers.

“The reefs cannot keep up with erosion without the help of those natural predators. Without action taken now, we risk allowing these reefs to erode past the point of no return,” van Woesik explained. 

Establishing stricter fishing regulations, including protected marine areas and no-take zones, can help fish populations rebound. Such conservation strategies have demonstrated success globally, creating conditions where marine ecosystems recover their natural balance.

Community involvement is equally essential. Educational campaigns and sustainable tourism initiatives can raise awareness about coral reef protection, ensuring local communities and visitors play an active role in conservation efforts.

Controlling sea urchins to save reefs

Hawaii’s coral reefs currently teeter near critical ecological thresholds. The scientific consensus clearly indicates urgent intervention is necessary to restore these fragile ecosystems.

Efforts to improve water quality, combat climate change impacts, and restore fish populations must operate simultaneously. Only through integrated, cooperative strategies can Hawaii safeguard its coral reefs for future generations.

The study is published in the journal PLOS One.

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