Influence of coral bleaching, coral mortality and conspecific aggression on movement and distribution of coral-dwelling fish

Changes in coral health may have significant impacts on fishes that shelter within live coral habitats. However, disturbances can be spatially patchy and it is unclear whether fishes subject to degradation of their immediate habitat are able to move and exploit alternative nearby healthy habitat patches. This study experimentally investigated the response of a common coral-dwelling damselfish (Dascyllus aruanus) to bleaching and mortality of host coral colonies. Three factors were considered that may influence the initiation and success of movement among habitat patches; i) condition of the host colony, ii) the availability of preferred habitat, and iii) the presence of conspecifics on alternative coral habitat. D. aruanus remained associated with bleached coral colonies and only moved to alternate habitats once the host coral died. On colonies that suffered complete mortality, 67% of associated fish vacated and migrated to healthy coral colonies. The remaining individuals persisted on dead colonies as a result of competition from aggressive resident fish on neighbouring healthy colonies, but relocated once offered an alternative colony without any competitive resident fish. When relocating in degraded reefs, 64% of the fish were successful in migrating to alternative healthy habitats. For these fish the selection of new habitat was based mostly on the presence of conspecifics, with 91% of fish relocating to healthy coral colonies containing conspecifics. The importance of conspecifics presence was further demonstrated by tank experiments where fish selected to associate with habitat containing conspecifics over unoccupied dead or healthy habitat. Movement of coral-dwelling fishes following disturbances will allow individuals to mediate the impact of host coral degradation by relocating to more suitable habitats. However, the potential for relocation will decline as disturbances become more severe and widespread, thereby increasing the distance to suitable habitats and reducing the availability of preferred habitat types. Furthermore, increased densities within habitat patches may increase competitive effects, with potential consequences for growth, survival and reproductive success.