Clownfish anemone bleaching changed the behaviour of clownfish, according to new research published on 11 November 2026 by Newcastle University. Marine heatwaves and anemone bleaching are altering clownfish behaviour, making the fish more defensive of their host and more aggressive towards one another. The study, published in the journal Royal Society Open Science, explores how clownfish behaviour changed during the fourth global bleaching event, the most extensive mass coral bleaching event on record.
Led by Newcastle University, the study was based on video recordings made during a marine heatwave in Kimbe Bay, Papua New Guinea, that lasted from March to July 2023. A group of Newcastle students, led by doctoral candidate Fletcher Noble, measured fish behaviour from the recordings, sorting actions into five categories: maintenance and defence, which help the anemone partner, and aggressive, neutral and submissive behaviours, which maintain the social group. The method allowed the team to see how time was spent before, during and after bleaching.
Clownfish
The results showed that, while higher temperatures alone had only a small effect on behaviour, the bleaching response of the anemone changed how the animals spent their time. Bleaching led to more defensive behaviours, which could benefit the host anemone, and aggression between group members increased by two hundred and fifteen per cent. The fish put their anemone home before their social group when bleaching struck, spending more time tending and guarding the anemone and less time on calm social interaction.
The study demonstrates that bleaching affects behaviour more than thermal stress, with heat stress having no impact on the activity of clownfish, which remained stable as temperature dropped through the heatwave. The finding matters because it shows that the effects of warming oceans go beyond physiology, changing how animals behave and interact. Such changes could affect survival, reproduction and the stability of reef communities.
The research was featured in Blue Planet III, the BBC ocean series narrated by Sir David Attenborough, giving the science a wide audience. Senior author Dr Theresa Rueger said the work shows how a bleaching event can reshape the daily life of a reef fish, and why protecting reefs from heat stress matters for the animals that live there. The study will now inform further work on how clownfish and anemones cope with a warming ocean.
Source: Newcastle University
