Ferocious fish losing their fight

Notoriously aggressive butterfly fish have been found to calm down after mass coral bleaching, according to a new study.

Coral reefs are collections of living corals which support thriving underwater ecosystems of diverse sea creatures. They are not plants but they are found in shallow waters because they live off tiny algae, which depend on the sunlight for photosynthesis. Reefs are vital for harmonising life under the sea, they provide stability to the sea bed, act as a water filter, and are a safe place for fish to store their unhatched eggs. These vibrant coral communities can be up to 10,000 years old, however, they are under threat from changing conditions in the sea, which will have devastating consequences on the dependent marine life and even the survival of our planet.

Rising sea temperatures due to global warming triggered a mass episode of coral reef bleaching in 2016, leading to the death of 30% of the coral in the Great Barrier Reef in Australia. The disruption to the ecosystems and the loss of food for some species is undoubtedly a significant problem. An example of a coral-dependent member is the territorial butterfly fish who actually eats coral to survive and will chase away any fellow fish trying to take a nibble of their patch.

However, their punchy attitudes changed after the global bleaching event and a team of researchers, led by Sally Keith from Lancaster University, observed that the butterfly fish became less aggressive as a result. Butterfly fish are sensitive to the change in reef condition and are, therefore, useful to study to reflect how other marine life respond to environmental change such as coral bleaching. They spent over 600 hours underwater logging 5,259 butterfly fish encounters.

But how can you measure the attitude of fish? An individual fish was watched for five minutes and any encounters with another butterfly fish were recorded as either friendly or aggressive. Before the coral bleaching event, Sally Keith and her team found that 15% of the fishy encounters were aggressive and resulted in a territorial telling off. However, after the coral bleaching, the fish seemed relaxed with only 5% aggressive encounters. Sally believes that now “they have gone past the point of being hangry [hungry & angry]”. She adds, “there is so little left for them to eat that they don’t have the energy to invest in fighting".

If the fish cannot be bothered to protect their favorite, nutritious coral, their energy levels will continue to drop meaning that they are less able to swim away from predators, which could lead to a decline in butterfly fish population. The team will continue to study butterfly fish in the same locations over the next four years to see if the observed change in behaviour has actually affected the population. They are also intrigued to see if the typical, aggressive fish behaviour returns as the coral reef starts to recover.