The effect of rubble stability on coral settlement and recruitment

Coral reef rubble comprises detached, fragmented, dead coral skeletons that can be unstable and are widely considered to constitute unsuitable substrate for coral recruitment. However, the type, severity, and frequency of a disturbance can generate rubble of diverse sizes, morphologies, and configurations, creating varying typologies of rubble beds. These physical characteristics play a substantial role in determining whether rubble remains stable or is frequently mobilised. We ask how the type of substrate influences the density of corals settling and recruiting onto rubble varying in its level of stability. Within two types of rubble beds, loose and interlocked, and on a control hard carbonate substrate, we measured coral settlement (four months post-spawning) and later-stage recruitment (post-settlement survival within 11 months) on unstable, unfixed rubble and on stabilised, fixed rubble. Both stability and substrate type influenced settlement and recruitment. Settlement and recruitment were higher on fixed than unfixed rubble regardless of the rubble type. This suggests that rubble mobility increases mortality rates. However, effect sizes varied between rubble bed types, likely driven by differing rubble bed characteristics. On the unfixed rubble, settlement and recruitment were higher in the interlocked than in the loose rubble bed. Settlement and recruitment were also higher on the fixed rubble in the interlocked bed than in the loose rubble bed. Thus, environmental effects in the loose rubble appear more severe than the combined environmental and mobility issues associated with the interlocked rubble. While our results show that stability is a key driver of coral recovery, settlement and recruitment on fixed rubble were still lower in rubble beds compared to the hard carbonate reef. These results indicate that each substrate has distinct environmental factors that differentially influence both the settlement and recruitment of corals, even on stabilised rubble surfaces. A greater understanding of coral recruitment dynamics across various rubble bed typologies is important for the management of future reef intervention programs, as the cover of rubble increases on reefs.