Breakdown of butterflyfish (Chaetodontidae) territories associated with the onset of a mass coral bleaching event

1. Obligate corallivorous butterflyfishes (Chaetodontidae) have been suggested as indicators of the health of coral reefs. 2. There has been a call to relate specific changes in butterflyfish ecology and behaviour to identified stressors. 3. For the first time it is reported here that there is a breakdown of the normal rigid territorial butterflyfish behaviour at the onset of an intense, large-scale coral bleaching event in the Seychelles, western Indian Ocean. 4. Behaviour changes from fixed territory holding, and a mosaic of intra- and interspecific territories, to one of mass excursions by the whole obligate corallivore assemblage. 5. The two main species studied here, Chaetodon trifasciaius and C. trifasciatus, fed almost exclusively on Acropora spp. In the western Indian Ocean these species of coral are among the first to die during a bleaching event. 6. It is speculated that the early excursion behavioural response by these butterflyfishes is because their prime food resource is the first to perish. 7. It is recommended that baseline data be gathered on corallivorous butterflyfish territory size and rate of excursion on healthy, unstressed reefs. 8. Deviation from these baseline results, along with early bleaching corals such as Montipora spp. and branching Acropora spp., is highly likely to indicate the coming of a bleaching event. Copyright (c) 2005 John Wiley & Sons, Ltd.