Sea urchins threaten coral reefs in Hawaii. Humans are to blame, of course

The coral reef in Hōnaunau Bay, Hawaii, is a paradise for divers and a sanctuary for dozens of species of tropical fish and sea turtles. However, its existence is threatened by voracious sea urchins that are rapidly decimating the colorful corals. Unfortunately, their uncontrolled reproduction is a result of human activity.

Who eats whom: fish, sea urchins, and corals

The costs of disturbing biological balance are surprisingly high, and humanity pays more and more enormous bills with each passing decade. Scientists from North Carolina State University (NCSU) took a closer look at the ecosystem of coral reefs in Hōnaunau Bay, Hawaii. Their analyses revealed that the mass spread of sea urchins is directly linked to anthropogenic pressure.

Let’s start at the beginning – with the natural state. A coral reef is a living organism and one of the most fascinating and beautiful types of habitats in the world. Corals feed on organic matter drifting in the water or on zooplankton, and often benefit from a symbiotic relationship with algae called zooxanthellae. On the other hand, as a key part of the food chain, corals themselves fall prey to other underwater creatures such as fish, crabs, and snails.

In Hawaii, coral connoisseurs also include sea urchins, which are primarily herbivorous organisms but won’t say no to the soft arms of a polyp. In a balanced ecosystem, the damage is minimal because the population of sea urchins is kept in check by predatory fish from the triggerfish, wrasse, snapper, and seabream families. Unfortunately, due to overfishing around Hawaii, there was a 45 percent drop in fish biomass between 2008 and 2018.

Hawaiian corals are withering

The reduction of natural predatory fish populations has led to an unprecedented population explosion of sea urchins in Hōnaunau Bay. The study’s lead author, Kelly van Woesik from the North Carolina State University Center for Geospatial Analytics, explains: We are observing areas where there is an average of 51 sea urchins per square meter, which is the highest population density in the world.

The overgrown sea urchin population is responsible for the intense erosion of coral reefs, which are already in a dire state. Pollution and elevated ocean temperatures caused by climate change severely limit coral growth and reproduction, making it difficult for reefs to cope with the urchin invasion.

The scale of the problem is clearly shown by the numbers – specifically, the level of carbonate production, considered a key indicator of coral reef growth. In the 1980s, Hawaiian corals produced an average of 15 kg of carbonate per square meter, which, according to van Woesik, corresponds to the global standard for healthy populations. Current measurements show only 0.5 kg of carbonate per square meter.

The situation is critical because, according to researchers from North Carolina, to fend off the sea urchin attack, corals in Hōnaunau Bay need to maintain reef coverage of at least 26 percent, and in shallower areas, even 40 percent. Data collected by diving teams and aerial photographs indicate, however, that current coverage is already down to just 28 percent.

Coral reefs are more than a tourist attraction

Media reports about the declining health of coral reefs worldwide are not always taken seriously. Yet reef degradation goes far beyond the loss of natural beauty and diving sites. In Hōnaunau Bay and many other locations, corals form a critical barrier protecting island coastlines from marine erosion, absorbing up to 97 percent of wave energy. Additionally, as a habitat for many species of fish and crustaceans, they often provide the foundation for the livelihood and sustenance of local communities.

According to van Woesik, the priority is to improve fisheries management in order to rebuild populations of predatory fish species. If we don’t take action now, we risk the reef erosion surpassing a point of no return, the researcher adds.

main photo credit: Oregon State University/Flickr