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“A better chance for resilience”: Using hungry fish to conserve coral reefs

by NOAA Fisheries 24 Apr 15:47 UTC
A school of large parrotfish (Chlorurus frontalis) observed during fish surveys on a coral reef. These fish use their bird-like beaks to scrape away seaweed, preventing establishment of seaweed on reefs. © Kevin Lino / NOAA Fisheries

For her Ph.D. research on invasive lionfish, Tye Kindinger spent most days underwater, scuba diving at reefs in the Bahamas. She became familiar with individual fish that she would see while monitoring the reef.

Each one had different behaviors. Damselfish were bold and vehemently guarded their territories, while some grouper were particularly friendly—"like puppy dogs." But over time, some of the reefs that Tye studied started to bleach and die. The fish she was accustomed to seeing disappeared as well. "When you suddenly see a reef bleach and those fish go missing, it's heartbreaking and you miss your buddies when you're underwater. Now imagine you depend on those fish and reefs for food or your livelihood," Kindinger says.

From then on, Tye knew that her research needed to address how climate change was impacting coral reefs and what could be done to save them. Now, she works in NOAA Fisheries' Pacific Islands Fisheries Science Center and researches how reef fish help protect corals from the adverse effects of climate change.

This interview was condensed and edited for clarity.

How is climate change affecting coral reefs in the Pacific?

The biggest impact is ocean warming. With rising temperatures, there are increasingly more ocean warming events that bleach corals. Corals unable to recover from bleaching eventually die, degrade, and disappear altogether. And, given the scale of ocean warming, one warming event can be linked to the eventual disappearance of a substantial amount of reef.

Another issue for corals is ocean acidification. The ocean absorbs some of the excess carbon dioxide in the atmosphere. This has caused the acidity of the ocean to increase. As the water becomes more acidic, it reduces the amount of calcium carbonate available in the water that corals use to grow and repair their skeletons. As a result, it can be more difficult for corals to compete for reef space, and overall erosion of reef structure may increase.

What prevents corals from recovering from bleaching?

Seaweed, or macroalgae, competes with corals for space on reefs. As warming events stress corals and cause them to bleach, algae can overgrow and smother corals. This could potentially tip the balance in favor of algae dominating the overall reefscape. Once algae takes hold and occupies an increasing amount of seafloor space, it becomes increasingly difficult for corals to regain their competitive "footing." New corals are less likely to find available space to successfully settle and help regrow the reef.

How is excess algae causing problems for other creatures in the reef?

"Excess" is the key word here. Algae gets a bad reputation, but there's a healthy level of algae that's important to the ecosystem. Algae provides habitat and food, and certain animals will attach their eggs to them.

Corals provide the hard reef structure that's a key component of these reef ecosystems. For fish, having hide-y holes is key to survival. Corals provide crevices, branching overhangs, and ledges for fish to hide from predators. Marine species rely on corals in other ways too. Some species, like butterflyfish, feed on corals.

When seaweed growth goes unchecked and corals begin to die, organisms that depend on the reef may disappear from it, decreasing the overall biodiversity of that ecosystem.

What can be done about excess algae on coral reefs?

Ocean warming events are only increasing in frequency. We want to be able to give corals the best possible chance of resisting or recovering from bleaching. One of the ways we can support corals is to keep algae on reefs at bay. Luckily, there are reef fish that graze on algae and can help control the abundance of algae on reefs.

NOAA takes an ecosystem-based approach to conserving coral reefs. We've been researching the importance of algae-eating fish in enhancing the resilience of Pacific coral reefs.

In order to effectively manage grazing on reefs, we need to know what it means to have enough grazers, and which fish species are critical for keeping algae in check. We know that herbivorous fish do not all consume the same amount of algae. Larger fish are able to consume more algae per bite. Species also vary in the proportion of algae that makes up their diet. The way fish consume algae can also vary. For instance, parrotfish have hard beaks that remove part of the reef substrate when they take a bite of algae, creating new spaces for corals to potentially settle onto. Surgeonfish, known as the "sheep of the reef," graze on the upper parts of algae.

How are you assessing the importance of grazers for protecting reefs from algae overgrowth?

When thinking about algae control, we know that all herbivorous fish are not equal. I developed a database of equations that allows users with fish-survey data to calculate the amount of algae that an herbivorous reef fish consumes. To create this tool, I first collected information about how different herbivore fish species feed: how many bites a fish takes, the size of their bites, and how much algae is in their diet.

Then, I related these species-specific feeding metrics to fish size. So, if you see an herbivorous fish species swimming around a reef and estimate how big it is, you can calculate how much algae it removes from that reef.

I'm currently applying this tool to the wealth of NOAA's National Coral Reef Monitoring Program data, which tracks reef fish populations in the U.S. Pacific Islands. Now, I can determine which fish are contributing the most to the overall grazing that occurs on a reef and which are especially important for maintaining this essential ecosystem function.

How can resource managers use this tool to protect coral reefs?

I'm working with partners to apply this tool to Hawaii Monitoring and Reporting Collaborative's data. We're investigating whether we can identify levels of grazing that are needed to maintain healthy coral reefs in the Main Hawaiian Islands for resource managers. For example, we're hopeful this type of analysis can inform Holomua Marine 30x30 Initiative planning.

My hope is that any reef fish monitoring program will be able to use this tool to easily convert their local herbivore fish counts into estimates of grazing across reefs. This will save them time and money in determining candidate areas for reef conservation actions.

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