Coral bleaching starves fish, leaving them too weak to fight

The 2016 global coral bleaching event made some reef fish too weak to fight for food, researchers speculate, after seeing a major reduction in aggression between members of territorial, coral-eating species.

Scientists led by Sally Anne Keith, a marine biologist at Lancaster University, UK, documented 3828 encounters between 38 species of butterflyfish – from the family Chaetodontidae – on 17 reefs in the central Indo-Pacific region, before and after the bleaching event.

“Aggression between butterflyfishes decreased by two-thirds following large-scale coral mortality, despite no significant change in fish abundance or community composition, they report in the journal Nature Climate Change.

Worldwide, coral bleaching has increased rapidly in the past 30 years, with events occurring five times more often. Reefs worldwide suffered a mass bleaching event in 2016 that scientists have linked to a rise in water temperatures caused by climate change. An estimated 30% of the coral on Australia’s Great Barrier Reef died during the event.

Butterflyfish are just one of scores of fish types that exist in reef ecosystems. They feed directly on the coral and are strongly territorial. Mated pairs usually defend their particular piece of coral, but it seems that the bleaching event disrupted normal behaviours. The researchers speculate that the change was due to the fishes’ nutritional deficit.

“Reduced aggression could indicate the breakdown of territories among butterflyfishes, as individuals roam further to obtain enough resources, rendering investment in aggressive defence too costly and potentially causing a shift from interference to exploitative competition”, they write.

They also suggest that such behavioural changes could be an early sign of population shifts.

Aggression was measured by incidents when butterflyfish came within one millimetres of another fish, either the same species or another that competes for coral resources.

But while the reduced aggression could have a short-term benefit, Keith’s team raises concerns about the long-term effects.

“Although behavioural flexibility can increase survival over the short-term, it can also create ecological traps in the longer-term if the disturbance endures, as is the case for mass coral mortality”, they write.

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