A Short-Term Winner? Dramatic Increases in the Population of Mushroom Coral Lobactis scutaria (Anthozoa: Fungiidae) in Kāne‘ohe Bay, Hawai‘i from 2000 to 2018

Abstract: Globally, coral reefs have been in significant decline for several decades. In contrast, the reefs in Kāne‘ohe Bay, Hawai‘i have presented an opposite trend during the same period, showing recovery and resilience even while experiencing significant disturbances. Since the diversion of sewage outfalls from southern Kāne‘ohe Bay in the late 1970s, coral populations have made a gradual recovery despite periodic mortality from heat-induced coral bleaching, competition from native and invasive algae, and freshwater inundation during heavy rainfall events. This study repeated surveys originally performed in 2000 for the free-living, solitary, common mushroom coral Lobactis (Fungia) scutaria to investigate population stability and size structure in the north, central, and south regions of the Bay. We found significant increases of up to 538% in the density of L. scutaria in Kāne‘ohe Bay between 2000 and 2018. An increased abundance of sexual recruits was particularly noteworthy. In 2018, the population structure reflected 9.2% sexual recruits and 3.8% asexual buds compared to 2000 when sexual recruits were nearly absent (0.4% of population.) We also found a positive linear relationship between sexual recruit density and adult density. Sites on patch reefs under management actions that involved the physical removal of invasive algae and sea urchin out-planting to crop the algae showed no difference in mushroom coral densities from control sites. While the growing population of mushroom corals in Kāne‘ohe Bay, Hawai‘i provides an exemplar species for resilience and recovery, it is unclear if this short-term expansion will continue under future conditions.