Herbivorous fish are crucial to the resilience of coral reefs

Herbivorous fish are crucial to the resilience of coral reefs

December 6, 2023

The study of fisheries management may have implications for public policy decisions

On December 8, the Hawaii State Board of Land and Natural Resources will decide on new rules governing fishing along the coast of the Hawaiian Islands.

It’s the first time in years that restrictions on commercial and non-commercial fishing have been considered at the state level, and comes after a two-year public process.

The package of herbivore fisheries rules is part of an effort to preserve the region’s coral reef ecosystems, which have been devastated by extreme human pressures such as overfishing and pollution, as well as climate change – particularly marine heat waves that have bleached coral and climate change. It led to widespread deaths.

Portrait of a woman wearing a black button-up shirt and long dark blonde hair

Assistant Professor Mary Donovan

Mary Donovan of Arizona State University studies the resilience of coral reefs, and the results of 10 years of research are outlined in a paper published today in the Proceedings of the Royal Society B.

“Coral reefs are on the front lines of climate change, and solutions are needed at both the local and global levels to limit their decline,” said Donovan, an assistant professor in the School of Geographical Sciences and Urban Planning. “We investigated how local management of herbivorous fisheries can improve coral reef resilience.”

“There’s always competition for space between corals and seagrasses,” said Russell Sparks, a Maui-area aquatic biologist. But rising marine temperatures have destroyed coral reefs and opened large areas for algae growth.

Sparks works for the Hawaii State Department of Land and Natural Resources, and his team provided much of the data collected for the study.

“If there are large numbers of herbivorous fish, they will control the growth of seaweed and stimulate conditions for new coral growth,” Sparks said.

The proposed restrictions include limiting the size and quantity of fish caught daily and annually.

Fisheries management is not a new idea. What is unique is the sheer volume of collective data that Donovan and her partners have provided as scientific evidence to support its necessity.

“This is what provided the weight of evidence that could be used for public policy,” she said.

Coral reefs are an essential component of the lives of Hawaiians who depend on them for protection from large ocean waves and to provide seafood and cultural identity. It is also a great attraction for tourists.

In recent years, climate change has resulted in the loss of nearly 50% of coral reefs in some areas of the islands, making herbivore-based management and coral reef resilience a priority for the State of Hawaii.

“This creates an increasing challenge for local reef managers, who are making difficult decisions for their communities,” Donovan said. “They are unable to control climate change, so they need tools to help build resilience into systems so that our coral reefs are able to withstand climate change. At least in the near term.”

Hunting for results

These tools came from research conducted through the Hawaii Monitoring and Reporting Collaborative, or HIMARC, which relied on more than 20,000 surveys throughout Hawaii.

The work produced results for every reef across the state — nearly 130,000 sites in total.

Donovan said the study would not have been possible without extensive cooperation from multiple agencies and institutions that monitor Hawaii’s coral reefs.

“This was a scientific leap,” Donovan said. “Combining data collected in different ways is a big challenge.”

The research has provided extensive evidence of the role that herbivores play in maintaining the balance between corals and algae on coral reefs and facilitating coral recovery and recruitment.

“Herbivores eat fleshy algae that compete with corals and thus serve an important ecosystem function,” Donovan said.

The study measured the impact of different metrics on catch numbers, with the largest negative impacts on those numbers coming from human drivers, such as commercial and non-commercial fishing using boats, spearfishing and netting.

Fish populations declined dramatically near some beaches, where pollution was another driver.

Factors affecting increased fish biomass were equally important. The largest amounts of fish are observed in remote areas, such as the Napali Coast of Kauai.

Specific types of fish—browsers, grazers, and skimmers—play important roles in coral reef resilience.

Browsers pick up large pieces of seaweed, grazers travel in large schools, constantly nibbling on algae, and skimmers (like parrotfish) do just that – they scrape the ocean floor, removing seaweed as well as coral tissue, which provides new space for corals to grow.

Research has demonstrated that populations below 80% of the natural local density of these fish threaten the resilience of coral reefs through overgrowth of algae consumed by the fish.

Move forward

The overall process of determining regulations has been long and arduous. The state must be as knowledgeable about local cultures, commercial and non-commercial fishing as it is about fisheries management.

If the new rules are approved this month, they will need the governor’s signature before they go into effect.

“Fishing is very important to the Hawaiian people, so the ecosystem role of herbivores must be balanced with maintaining access to fishing,” Donovan said. “The proposed rules are a step in the right direction, but they are just the beginning, and the work will continue as we support other island-wide and venue-wide management processes.”

Top photo courtesy Hawaii DLNR

Dolores Tropiano

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