Fish lose their color as coral reefs shrink

(ORDO NEWS) — Fish communities on Australia’s famous Great Barrier Reef may become less colorful as the ocean warms and coral bleaching occurs, according to a new Australian study that looks at changes in the reef’s health, coral types and fish populations over three decades.

“Future reefs may not be as colorful ecosystems as they are today,” write marine ecologist Chris Hemingson and colleagues at James Cook University in their paper, published in the journal Global Change Biology.

“Our results suggest that reefs may be at a critical transition point and could become much less colorful in the coming years.”

The study, weeks after the Great Barrier Reef was hit by another massive bleaching event caused by rising carbon dioxide emissions, focused on the reefs around Orpheus Island, which sits in the middle of the largest coral reef system on Earth.

Previous coral bleaching incidents have severely altered the composition of coral reefs in the area, and the loss of soft and branching corals has been a likely factor in the disappearance of increasingly rare, brightly colored fish, the study said.

“As these complex corals become rarer, fish communities may become dimmer on future reefs affected by climate change,” the trio of researchers wrote on their group’s website.

For the study, Hemingson and colleagues studied the variety of colors found in reef fish communities and linked this to the types of habitats these fish live in.

Whether fish have developed bright colors to stand out and attract a mate, or neutral tones to blend in with their surroundings and avoid predators, their coloration is naturally associated with the coral reefs they inhabit.

Fish communities in healthy parts of the reef with an abundance of complex corals were compared with other areas dominated by massive, encrusted corals formed after severe shocks such as heatwaves and cyclones, and with degraded reefs with few corals overgrown with algae.

“We found that as the amount of structurally complex corals on a reef increases, the variety and color range of fish living in and around them increases,” says Hemingson, who has focused on smaller fish that rarely swim far from their home reef.

“But as the cover of soddy algae and dead coral debris increases, the variety of colors diminishes to a more generalized, monotonous appearance.”

This does not bode well for reef fish in warmer waters, so Hemingson and his colleagues looked at data collected annually over the past 27 years on fish communities on the reefs around Orpheus Island to see if these trends continue over time.

We know from other previous studies that just 2 percent of the Great Barrier Reef remains untouched by five massive coral bleaching events in the past 30 years – a truly horrific impact of anthropogenic heat waves caused by atmospheric emissions.

The first recorded mass bleaching event in 1998 hit the reefs around Orpheus Island particularly hard, damaging complex branching corals and causing a “complete shift” in fish communities, Hemingson and colleagues found.

The most attractive yellow and green fish, such as lemon damselfish and green coral goby, have declined by about two thirds in the three decades since.

The continued decline in numbers (exacerbated by further disturbances) is likely to result in the total loss of these brightly colored species, “washing out color from fish communities, leaving them dull and dull,” write Hemingson and his coral reef colleagues.

While the massive and encrusted boulder corals that have replaced soft branching corals are more resistant to heat, hardening the reef for future stressors, they are likely to provide less protection from predators for the brightly colored fish.

“Unfortunately, this means that the corals most able to survive the immediate effects of climate change are unlikely to be able to maintain the diversity of colors currently supported by coral reefs,” the researchers write.

“Therefore, fish communities on future reefs are likely to be a dimmer version of their former configurations, even if coral cover remains high.”

Hemingson acknowledges that people can experience grief over the loss of fish and coral species – an ecological grief that is well known to coral reef scientists first-hand.

But as scientists have conclusively proven, grief can be a powerful motivating force that spurs people to action.

“I can either give up when I feel frustrated or use those emotions to motivate me and find better solutions,” Emma Camp, a coral biologist at the University of Technology Sydney, told Nature magazine in 2019.

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