Two years ago, pictures of bleached corals at the Great Barrier Reef in Australia shocked the world. A 9 month marine heat wave had caused global coral bleaching, leading to devastation of coral reefs across the planet. Bleaching occurs when corals are stressed by changes in temperature, light or nutrients. When faced with this challenge, the algae living inside the corals depart, causing the corals to turn completely white. Bleaching does not mean that all of the corals are dying, but as they feed on sugars produced by their algal lodgers, they often struggle without them. As the reefs are important underwater ecosystems providing breeding grounds and food sources for many fish species, Sally Keith from Lancaster University, UK, together with an international team of collaborators, wanted to know how coral mortality and fish behaviour are linked.
In recent years, the team has investigated reef fish behaviour on 17 reefs within the Indo-Pacific Ocean, including 10 that they studied before and after the 2016 bleaching event. On each occasion, they observed individual butterflyfish for 5 min periods, noting each encounter with other fish as well as how frequently the fish bit at corals. In addition, they compared the number of healthy corals before and after the heat wave and found that bleaching caused a loss of between 18% and 65% of corals across 13 reefs distributed across four regions in Indonesia, Japan and Christmas Island. However, the team also identified a fifth region in the Philippines where bleaching did not lead to the death of the corals. This allowed them to compare how the fish behaved in healthy and unhealthy reefs and revealed dramatic differences in the animals’ behaviours.
Unexpectedly, the butterflyfish appeared to become less aggressive towards other fish in reefs that experienced coral death; the scientists observed up to 85% fewer bites at the fish's preferred corals and the fish did not compensate for this reduction by feeding more on the surviving coral species. Importantly, the bite rates on preferred corals in the reefs in which all corals survived declined by only 7% and the fish's aggression rates also changed only slightly, convincing the team that their findings were indeed related to the death of the coral. The researchers also ruled out that the changes they saw were caused by a reduction in the number of fish, as that did not differ before and after the bleaching event. Instead they think that the decrease in aggression is caused by a reduction in the availability of food. Aggressive encounters between fish species are linked to individuals defending the coral colonies in their territories, but hungry fish do not have a lot energy to fight and there might not be enough healthy corals left to justify the effort.
These behavioural changes are likely to be one factor explaining why fewer butterflyfish species are found in coral reefs in the 5 years following mass coral die offs. After losing their favourite staple, the fish will have to swim further to find food, which can eventually lead to them giving up their territories and abandoning ruined reefs.