Bleached reefs still support nutritious fish, study finds

Escalating ocean temperatures stemming from climate change are devastating the world’s tropical coral reefs. In response to the stress, corals, which are animals, sometimes unceremoniously jettison the algae that live within them. That expulsion drains the color from the reefs in what’s known as bleaching. In the severest cases, it can kill the coral, which need the algae to provide them with nutrients, oxygen and waste management.

At the same time, millions of people in the tropics eat fish that live on these reefs. And today, the widespread bleaching of tropical reefs, which is expected to continue as the Earth heats up, has thrown into question how those fisheries and the communities that depend on them for sustenance will respond.

Now, a new study published Jan. 6 in the journal One Earth has found that in certain circumstances, critical nutrients for human development found in reef fishes remain available even after mass bleaching has occurred.

‘An important message here is that climate-impacted reefs can still provide some important ecosystem services, and therefore should still be considered in management plans and conservation,’ said Camille Mellin, a quantitative ecologist at the University of Adelaide in Australia, who was not involved in the study.

James Robinson, the study’s lead author and an ecologist and research fellow at Lancaster University in the U.K., said the 1998 bleaching of coral reefs in Seychelles, an island nation in the Indian Ocean off the eastern coast of Africa, makes for a ‘really good case study.’ The bleaching killed more than 90% of live corals. Around 60% of Seychelles’ reefs recovered after the event. The other 40% were taken over by seaweed.

Since the bleaching, Robinson and his colleagues have probed how the bleaching and subsequent fallout have affected the fisheries anchored by these reefs. Globally, 6 million people fish coral reefs, according to a 2013 study, and millions more rely on the production of these fisheries for food. The calories they provide, along with macronutrients like protein and lipids, are essential — the average Seychellois gets almost half of the animal protein they consume from fish. But eating fish also supplies a host of the micronutrients critical to the healthy functioning of the human body, Robinson said in an interview.

Given their prior research, ‘It was a sort of natural next step to say, how do we put a nutrient lens on what we know about Seychelles, and what can we learn about how bleached reefs might be contributing to people’s diets?’ he added.

To answer those questions, the researchers sampled fish from both seaweed-covered and recovering reefs in 2017, and later, in 2019, they teamed up with local fishers to do the same. They also used data from underwater surveys that team members collected between 1994 and 2017 to identify the fish that inhabit the islands’ reefs.

The scientists used laboratory analyses and mathematical modeling to sort out the concentration of certain micronutrients important to human growth and development — calcium, iron, selenium, zinc, and omega-3 fatty acids — in 43 species of coral reef fish, which represent those targeted by fishers in Seychelles and elsewhere in the tropics. They then used their models to predict nutrient concentrations across other species. By leveraging data from before and after the 1998 bleaching event, they compared how the levels of nutrients available from the collection of fish living on the reef may have changed in response to the change in the surroundings.

The team’s analysis found that the availability of the five micronutrients in fisheries persisted after bleaching, and in some cases, it even increased. The researchers think that’s because plant-loving grazers, such as zinc- and iron-rich parrotfish (family Scaridae) stuck around or were more numerous on bleached reefs colonized by seaweed.

Reef fish in general also supply at least as much of these nutrients as other animal sources such as livestock.

‘While most scientific studies of climate impacts on natural ecosystems have highlighted negative impacts, this study shows that some ecosystem services (here, supply of five micronutrients) can be maintained despite climate change and coral bleaching, which was surprising (and good news!),’ Mellin said in an email.

Moreover, the findings reveal that different nutrients are cached in separate parts of the reef food chain. For example, more carnivorous fish tend to have higher levels of calcium, selenium and omega-3 fatty acids than herbivores, Robinson said. That emphasizes the importance of including not just fish in general, but also a diversity of species, in human diets. It also points to the need for different fishing methods to tap into the full suite of micronutrients available in fish flesh, because fishers in Seychelles often use traps to catch herbivorous fish and handlines to catch carnivorous ones.

The results could be relevant for fish-reliant communities elsewhere.

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‘Fishing has really been a cornerstone of food security in many parts of the world,’ said Johann Bell, senior director of tuna fisheries with Conservation International, who was not part of this study.

Bell’s research in the 2000s focused on the sources of protein in islanders’ diets. Fish, he and his colleagues found, account for 50-90% of animal-based protein in the Pacific Islands, where, like in Seychelles, many communities rely on coral reef fisheries.

But today, the focus of research on nutrition related to coral reefs has shifted, he said.

‘Quite rightly,’ Bell said, ‘there’s been a real emphasis on the micronutrients because, where they are lacking in diets, they can really have very deleterious effects.’

He said the study was ‘an important piece of work’ and ‘well done.’

Diets without enough of the minerals looked at in this study, and other nutrients such as vitamins, contribute to what coral reef ecologist Kirsty Nash calls a ‘hidden hunger.’ These micronutrients are vital for things like brain development and are especially important for children and pregnant women.

Nash, a research fellow at the University of Tasmania, called the research ‘super exciting.’ She noted that teasing apart the effects of changes that occur after bleaching is critical.

‘Understanding what those novel ecosystems are going to provide to us is a really important step,’ Nash said.

Still, she said, more research is needed to sort out the consequences of bleaching on a broader scale.

‘There’s a range of other potential outcomes of bleaching that could affect the fish community in a different way,’ Nash said. For example, bleached reefs aren’t always taken over by seaweed.

‘Some of them just end up in rubble,’ she added.

And questions remain about the impact of successive bleaching, as is expected with climate change over the coming century. Seychelles had another mass bleaching event in 2016 at the tail end of the data set used in this study. Robinson said the team hoped to return to Seychelles soon to determine the impact of this latest event.

Prior research has shown that bleaching winnows the diversity of fish species found on a reef — diversity that, as this study shows, plays a vital role in food security for human communities. Bell noted that warmer overall water temperatures have been shown to tinker with fish physiology, potentially impeding their growth and spawning.

Those effects in tandem with bleaching could force communities to change the fish species they target, and it highlights the need to incorporate their dietary requirements into discussions about where to focus efforts to protect reefs, he said.

‘We need to conserve the biodiversity,’ Bell said, ‘but we need to do it in a way where they can continue to use it.’

Banner image of Victoria Market in Seychelles by John C. Cannon. 

John Cannon is a staff features writer with Times of Nation. Find him on Twitter- @johnccannon

Citations-

Bell, J. D., Ganachaud, A., Gehrke, P. C., Griffiths, S. P., Hobday, A. J., Hoegh-Guldberg, O., … Waycott, M. (2013). Mixed responses of tropical Pacific fisheries and aquaculture to climate change. Nature Climate Change, 3(6), 591-599. doi-10.1038/nclimate1838

Robinson, J. P., Maire, E., Bodin, N., Hempson, T. N., Graham, N. A., Wilson, S. K., … Hicks, C. C. (2022). Climate-induced increases in micronutrient availability for coral reef fisheries. One Earth. doi-10.1016/j.oneear.2021.12.005

Teh, L. S., Teh, L. C., & Sumaila, U. R. (2013). A global estimate of the number of coral reef fishers. PLOS ONE, 8(6), e65397. doi-10.1371/journal.pone.0065397

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