Comparing Coral Demographic Surveys From In Situ Observations and Structure-From-Motion Photogrammetry on High Diversity Reefs Shows Low Methodological Bias but Highlights Persistent Areas of Concern

The Pacific Islands Fisheries Science Center’s Ecosystem Sciences Division (ESD) executes the National Coral Reef Monitoring Program (NCRMP) across 40 primary islands, atolls, and shallow banks in the U.S. Pacific. The goal of NCRMP is to understand how reefs are responding, in both time and space, to local and global disturbances, and what factors enhance reef resilience.

Assessments of benthic cover and coral demography are key components of ESD’s Pacific NCRMP and have historically been conducted using in-water visual surveys and photoquadrats by a team of three to four divers. With NOAA ship time becoming more limited and increasing complexities of field missions, Pacific NCRMP has been exploring innovative technologies to more efficiently monitor benthic communities.

A primary focus of this effort has been Structure-from-Motion photogrammetry (SfM), which generates 3D reconstructions of a scene from overlapping images. In 2021, we published a study comparing data generated from in-water surveys to SfM-derived metrics for assessing coral demography in the low diversity reefs of the Main Hawaiian Islands (Couch et al., 2021).

The goal of that study was to evaluate whether NCRMP can maintain continuity in our long-term NCRMP data sets if the program transitions to SfM survey methods. Couch et al. (2021) presented concerns about the comparability and continuity of estimates of partial mortality and bleaching prevalence between methods as well as high inter-observer error for both methods. As such, ESD modified existing protocols and training which improved consistency between observers in sizing colonies and estimating percent mortality (known as the “lumping-splitting challenge”). In the current study, we reevaluate the comparison on higher diversity reefs in the Mariana Archipelago and American Samoa.

The three objectives of this study:
1) Compare error between methods to within-method observer error,
2) Test for methodological bias between SfM and in-water visual surveys, and
3) Provide recommendations for the transition to SfM for future NCRMP benthic monitoring through a synthesis of Couch et al. (2021) and the present study.

Charendoff JA, Couch C, Oliver T, Lamirand M, Amir C, Basden I, Torres-Pulliza D, Asbury M, Winston M, Huntington B, Lichowski F, Osborn N, Ehrenberg J, Garriques J, Samson J 2023. Comparing coral demographic surveys from in situ observations and structure-from-motion photogrammetry on high diversity reefs shows low methodological bias but highlights persistent areas of concern U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-153, 52 p.
https://doi.org/10.25923/8r7f-t404