How coral bleaching is causing fish to fight losing battles

Butterflyfish swimming in coral
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Fish that have lost food due to mass coral bleaching are getting into more unnecessary fights, causing them to expend precious energy and potentially threatening their survival, according to new research.

The climate crisis is threatening the future of the world’s coral reefs. As the Revelator has reported, one-third of all known species of coral are now endangered, with global warming being the primary factor in their predicament in many cases. Coral reefs support around 25% of ocean life, despite accounting for less that 1% of the ocean floor. As such, “entire ecosystems could collapse” if they disappear, the conservation-focused publication warned.

Devastating – but a perfect opportunity

Sally Keith, a marine ecologist at Lancaster University, and her colleagues had no idea a mass bleaching event was coming when they first studied fish at 17 reefs off Japan, the Philippines, Indonesia, and Christmas Island.

But Keith told Agence France-Presse that when one of history’s worst global bleaching events struck in 2016, it offered “the perfect opportunity” to study how it affected the behaviour of 38 species of butterflyfish. She explained that the colourfully-patterned reef fish are the first to feel the effect of bleaching because they eat coral, so their “food source is hugely diminished really quickly”.

The researchers returned within a year and were “shocked” to see the devastation of the once-beautiful reefs, she said. Donning their snorkels or scuba gear, the team watched the fish “swimming around looking for food that just isn’t there anymore”.

A losing battle

The bleaching particularly affected Acropora coral, the main food source for the butterflyfish. That “changed the playing field of who’s eating what”, Keith said, putting different species of butterflyfish in increased competition for other types of coral.

When a butterflyfish wants to signal to a competitor that a particular bit of coral is theirs, they point their noses down and raise their spiny dorsal fins. Keith explained:

Read on...

It’s almost like raising your hackles

If that fails, one fish will chase the other, usually until the other gives up.

The team observed 3,700 encounters between butterflyfish for the new study, which was published in the journal Proceedings of the Royal Society B. Before the coral-bleaching event, different species of butterflyfish were able to resolve disputes using signalling around 28% of the time. However, according to the study, that number fell to just 10% after the bleaching, indicating many “unnecessary attacks”.

In other words, the research suggests that bleaching events are frustrating fishes’ ability to identify competitors. According to Keith:

By recognising a competitor, individual fish can make decisions about whether to escalate, or retreat from, a contest – conserving valuable energy and avoiding injuries.

Tipping fish over the edge

Keith, who is the study’s lead author, explained the impact of the fish’s changed behaviour:

Making poor decisions about who to fight, and where to invest their really valuable energy, could be that little bit that tips them over the edge towards actual starvation

It isn’t clear if the fish will be able to adapt to the changes brought about by coral bleaching quickly enough, the researchers warned. This could also have knock-on effects between species and up the food chain.

Human-driven climate change has spurred mass coral bleaching as the world’s oceans get warmer. Modelling research last year found that even if the Paris climate goal of holding global warming to 1.5 degrees Celsius is reached, 99% of the world’s coral reefs will not be able to recover. At two degrees of warming, that number rises to 100%.

The loss of reefs is devastating to marine and human communities. As the Pew Charitable Trusts have highlighted, over one billion people depend on reefs for their livelihoods. Moreover, coral reefs provide protection from extreme weather such as storms and flooding that is becoming increasingly common as the world heats up. Without them, research suggests that the damage from floods and storms will rise steeply.

Additional reporting by Agence France-Presse

Featured image via Paul Asman and Jill Lenoble / Wikimedia, cropped to 770×403, licensed under CC BY 2.0

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