The Hawaiian Islands Humpback Whale National Marine Sanctuary (HIHWNMS) is jointly managed by the National Oceanic and Atmospheric Administration (NOAA) and the State of Hawaii’s Department of Land and Natural Resources. While currently focused on protecting the North Pacific Humpback Whale (Megaptera novaeangliae), the Sanctuary is also home to a variety of seabirds, turtles, seals, other marine mammals and coral reef ecosystems. In response to public comments, the HIHWNMS is exploring the expansion of its scope to include the conservation and management of these other ecosystem components. The HIHWNMS reached out to the Biogeography Branch of NOAA’s National Centers for Coastal Ocean Science (NCCOS) Center for Coastal Monitoring and Assessment (CCMA) to better understand the distribution of these resources, beginning with coral reefs at moderate depths. These moderate depth or “mesophotic” corals are important because they are understudied, under threat and thought to produce benefits similar to those provided by shallow-water coral ecosystems. Our primary goal was to develop predictions of mesophotic coral distributions in a select region of the Sanctuary and to share these best practices with HIHWNMS staff to support their internal review process. A pdf of the final report titled, Prediction of Mesophotic Coral Distributions in the Au‘au Channel, Hawaii, as well as other details about this project are available online. HIHWNMS Superintendent, Dr. Malia Chow also sat down with us to discuss this project. That interview will be posted next Monday.
Our Study Subject: Mesophotic Hard Corals
Mesophotic corals (or Mesophotic coral ecosystems – MCEs) typically live between 30 to150 meters in depth. MCEs show a range of adaptations which allow them to live in these low light environments, including flattened shapes, pigment specialization, increased heterotrophy and lower metabolic demands.Several other physical factors also influence the distribution of mesophotic corals, including aragonite saturation, alkalinity, pH, currents, water temperature and the availability of hardbottom.
Predicting Coral Distribution
Maximum Entropy (MaxEnt) modeling software was used to create four separate maps of predicted habitat suitability for: (1) all mesophotic hard corals combined, (2) Leptoseris, (3) Montipora and (4) Porites genera. MaxEnt works by analyzing the distribution of environmental variables where species are present, so it can find other areas that meet all of the same environmental constraints. Approximately 2,000 records containing information about mesophotic coral occurrence and 34 environmental predictors describing the seafloor’s depth, vertical structure, available light, surface temperature, currents and distance from shoreline were used to train MaxEnt. These datasets were collected by a number of organizations, including NOAA’s Pacific Island Fisheries Science Center, the Hawaii Undersea Research Laboratory, the Bishop Museum, the Hawaii Division of Aquatic Resources, the University of Hawaii, the National Aeronautics and Space Administration, and the U.S. Geological Survey. NOAA’s Center for Sponsored Coastal Ocean Research, and Office of Marine Aviation Operations funded the collection of the mesophotic coral data.
To understand the accuracy of these predictions, we calculated the number and ratio of correctly predicted coral presences to correctly predicted coral absences. All maps were between 73-86% accurate. Additional analysis revealed depth, distance from shore, average available light at depth, and variances of available light at depth were especially important. Our findings suggested that mesophotic corals generally prefer moderately deep waters that remain clear and stable through time. It is also highly likely that historical events (e.g., the coral harvesting for jewelry) and several other biological and ecological factors (predation and competition), play a significant role in shaping coral distributions.
Potential applications of these predictive models
Managers must understand the spatial distribution of habitats and organisms before they can devise appropriate strategies to manage them. Where are the important habitats found? How abundant are species of concern? What threats are located near key resources? How do key resources overlap with each other? Results from this study can be used to answer some of these questions and to support a number of management actions, including identifying large areas of high suitability by coral genus; delineating subzones within the sanctuary if special regulations are needed to protect MCEs; and targeting and promoting research and educational activities about these important and rare habitats. These predictive maps may also be overlaid with additional spatial information (e.g., human activities) to evaluate options for minimizing conflicts in areas with many overlapping resources and uses. Establishing a baseline for MCEs distributions in the MHI is critical because it will allow scientists and resource managers to better understand how MCEs are responding to local environmental variations and global climate changes in the future.