Dramatic Increases In The Mushroom Coral Lobactis Scutaria Population In Kāneʻohe Bay, Hawaiʻi Over The Last 18 Years

Abstract

The coral reefs of Kāneʻohe Bay, Hawaiʻi have undergone significant changes in recent history. By the 1960s, vibrant coral reefs were transformed to an algae-dominated ecosystem as a result of dredging and increased nutrient input from wastewater discharge. Since the diversion of sewage from the southern bay in the late 1970s, coral populations have made a slow recovery despite periodic mortality from recent coral bleaching and freshwater flooding during heavy rainfall events. This study repeated surveys done in 2000 for the common mushroom coral Lobactis scutaria, previously referred to as Fungia scutaria. Unlike most coral species, L. scutaria is a solitary, free-living, non-colonial coral such that each coral is a single polyp which makes them easy to identify and count in the field. At 42 sites on patch reefs across Kāneʻohe Bay, two snorkelers surveyed L. scutaria density and colony size during 30 minute-timed swims at the same general locations of the 2000 study. Surveyed areas during the timed swims were recorded with GPS tracks and converted to total area surveyed for each patch reef site to calculate coral density. I found a significant increase in density of L. scutaria for all regions of the bay between 2000 and 2018 but coral size structure did not change significantly. The North region of Kāneʻohe Bay contained nearly twice the recruit density as the other regions and there was a positive relationship between sexual recruit density and adult density. In 2018, sexual recruits made up 9.2% of the population while asexual buds made up 3.8% of the population, showing a shift in reproductive modes from 2000 when sexual recruits were nearly absent and asexual buds constituted about 20% of the population. Sites on patch reefs under restoration by the Hawaiʻi Department of Land and Natural Resources that involved removal of invasive algae and sea urchin out-planting showed no difference in mushroom coral densities than control sites. This is probably due to a decrease in algae bay-wide and a recovery of coral on all reefs, not only reefs that had restoration activities. These findings indicate a growing population of mushroom corals and recovering coral reef ecosystem in Kāneʻohe Bay, Hawaiʻi.