Fish that have lost food due to mass coral bleaching are getting into more unnecessary fights, using up precious energy and potentially threatening their survival, new research found yesterday.
With the future of the world’s coral reefs threatened by climate change, a team of researchers studied how a mass bleaching event affected 38 species of butterflyfish.
The brightly patterned reef fish are the first to feel the effects of bleaching because they eat coral, so their “food source is declining tremendously very quickly,” said Sally Keith, a marine ecologist at Britain’s Lancaster University.
Little did Keith and her colleagues know a mass bleaching event was imminent when they first studied the fish on 17 reefs off Japan, the Philippines, Indonesia and Christmas Island.
But when 2016 saw one of the worst global bleaching events in history, it presented “the perfect opportunity” to study how it affected fish behavior, Keith told AFP.
The researchers returned within a year and were “shocked” to see the devastation to the once-beautiful reefs, she said.
The team donned their snorkels or scuba gear and watched the fish “swim around looking for food that just isn’t there anymore,” she added.
“There was a little crying in our masks.” Battle Lost Acropora coral, the butterflyfish’s main food source, was particularly affected by the bleaching.
That “changed the playing field of who eats what,” Keith said, putting different types of butterflyfish in increased competition with other types of coral.
When a butterflyfish wants to signal a competitor that they own a particular piece of coral, they point their nose down and raise their spiny dorsal fins.
“It’s almost like raising the hairs on the back of your neck,” Keith said. If that fails, one fish will chase the other, usually until the other gives up.
“I followed one [fish] for about 50m once; it was pretty tough, they’re very fast,” said Keith. The team observed 3,700 encounters between butterflyfish.
Before coral bleaching, different butterflyfish species could use signals to resolve disputes about 28% of the time.
But that dropped to just 10% after the bleaching event, prompting many “unnecessary attacks,” according to the new study in the journal Proceedings of the Royal Society B.
“Making bad decisions about who they fight and where they put their really precious energy could be the little clue to them [the fish] get past the edge of actual starvation,” said Keith, the study’s lead author.
The researchers warn that it is unclear whether the fish can adapt quickly enough to the changes caused by coral bleaching.
Bleaching could also have knock-on effects between species and also in the food chain, she added. Human-caused climate change has led to massive coral bleaching as the world’s oceans get warmer.
Modeling research last year found that even if the Paris climate target of keeping global warming to 1.5°C is met, 99% of the world’s coral reefs will fail to recover. Two degrees of warming would increase that figure to 100%.