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Herbivore fish crucial to coral reef resilience

Fishery management study may have implications for public policy decisions


Brighlty colored fish swimming along a coral reef.
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December 06, 2023

On Dec. 8, the Hawaii State Board of Land and Natural Resources will decide on new rules regulating fishing along the coasts of the Hawaiian Islands.

It’s the first time in years that restrictions on statewide commercial and noncommercial fishing will be considered, and comes after a two-year public process. 

The herbivore fisheries rule package is part of an effort to preserve the area’s coral reef ecosystems, which have been devastated by severe human stressors such as overfishing and pollution, as well as climate change — particularly marine heat waves that have bleached away the coral color and led to massive mortality.  

Portrait of woman in black button up shirt and long dark blonde hair

Assistant Professor Mary Donovan

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

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

Russell Sparks, a Maui district aquatic biologist, said, “There is always a competition for space between coral reefs and seaweed. But increased marine temperatures have destroyed coral reefs and opened up large spaces for algae to grow.”

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

“If there are abundant populations of herbivorous fish, they will control the growth of seaweed and stimulate conditions for new corals to grow,” Sparks said. 

Proposed restrictions would include limiting the size and the quantity of the fish caught daily and yearly.

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

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

Coral reefs are central to the lives of the people of Hawaii who rely on them for protection from large ocean waves, provision of seafood and cultural identity. They are also a major draw for tourists.

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

“This creates an ever-mounting challenge for local reef managers, who are making hard decisions for their communities,” Donovan said. “They are unable to control climates change, so they need tools to help build resilience in the systems such that our reefs might be able to withstand climate change. At least in the near term.”

Fishing for findings 

Those tools came out of the research conducted through the Hawaiʻi Monitoring and Reporting Collaborative, or HIMARC, which drew on more than 20,000 surveys around Hawaii.

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

Donovan said the study would never have been possible without extensive collaboration from multiple agencies and institutions that monitor Hawaii’s reefs. 

“This has been a scientific leap,” Donovan said. “Combining data collected in different ways is very challenging.” 

The research provided extensive evidence of the role that herbivores have in maintaining a balance between corals and algae on 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 various metrics on the quantity of fish, with the largest negative effects on those numbers coming from human drivers, such as commercial and noncommercial boat-based spear and net fishing.

The number of fish was drastically reduced near certain shorelines, where pollution was another driver.

Factors impacting the increase of the biomass of fish were of equal importance. The greatest quantities of fish were observed in more remote areas, such as the Napali coast of Kauai. 

Specific types of fish — browsers, grazers and scrapers — play significant roles in the reef resilience.

Browsers pick at big pieces of seaweed, grazers travel in big schools, constantly nipping at the algae, and scrapers (such as the parrotfish) do just that — they scrape the bottom of the ocean floor, removing seaweed as well as coral tissue, which makes new space for corals to grow. 

The research established that populations of less than 80% of the natural local density of these fish compromises reef resilience through the overgrowth of algae that the fish consume. 

Moving forward

The public process for determining regulations has been a long and arduous one. The state must be as mindful of local cultures and commercial and noncommercial fishing as they are of fishery management.

If the new rules are approved this month, they will need to be signed by the governor before going into effect.

“Fishing is incredibly important to the people of Hawaii, 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 it’s just the beginning, and the work will continue as we support other management processes at island- and place-based scales.”

Top photo courtesy Hawaii DLNR

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