Comparing Coral Colony Surveys From In-Water Observations and Structure-From-Motion Imagery Shows Low Methodological Bias

Abstract

As the threats to coral reefs mount, scientists and managers are looking for innovative ways to increase the scope, scale, and efficiency of coral reef monitoring. Monitoring changes in coral communities and demographic features provides key information about ecosystem function and resilience of reefs. While most monitoring programs continue to rely on in-water visual survey methods, scientists are exploring 3D imaging technologies such as photogrammetry, also known as Structure-from-Motion (SfM), to enhance precision of monitoring, increase logistical efficiency in the field, and generate a permanent record of the reef. Here, we quantitatively compare data generated from in-water surveys to SfM-derived metrics for assessing coral demography, bleaching, and diversity in the main Hawaiian Islands as part of NOAA’s National Coral Reef Monitoring Program. Our objectives were to compare between-method error to within-method error, test for bias between methods, and identify strengths and weaknesses of both methods. Colony density, average colony diameter, average partial mortality, prevalence of bleaching, species richness, and species diversity were recorded using both methods within the same survey areas. For all metrics, the magnitude of between-method error was comparable to the within-method error for the in-water method and between method error was significantly higher than within-method error for SfM for one of the seven metrics. Our results also reveal that a majority of the metrics do not vary significantly between methods, nor did we observe a significant interaction between method and habitat type or method and depth. Exceptions include estimates of partial mortality, bleaching prevalence, and Porites juvenile density–though differences between methods are generally small. Our study also highlights that SfM offers a unique opportunity to more rigorously quantify and mitigate inter-observer error by providing observers unlimited “bottom time” and the opportunity to work together to resolve difficult annotations. However, the necessary investment in equipment and expertise does present substantial up-front costs, and the time associated with curating imagery, photogrammetric modeling, and manual image annotation can reduce the timeliness of data reporting. SfM provides a powerful tool to reimagine how we study and manage coral reefs, and this study provides the first quantified methodological comparison to validate the transition from standard in-water methods to SfM survey methods for estimates of coral colony-level surveys.

Comparison of Standard Caribbean Coral Reef Monitoring Protocols and Underwater Digital Photogrammetry to Characterize Hard Coral Species Composition, Abundance and Cover

“…6), is lower than those reported from The standardization of underwater photogrammetry protocols needs to be considered. In a recent study, Couch et al (2021) detected no differences on the estimation of coral Relative coral cover range in the graphs is between 0 and 2.83. The colors represent each protocol.…”

“…Urbina-Barreto et al (2021) showed that the LIT overestimates coral cover in comparison to UWP. However, Couch et al (2021) observed a high consistency of data between UWP and field visual surveys. Considering the advantages and recent integration of underwater photogrammetry to monitor coral reefs, there is a clear need to compare its performance with commonly used protocols.…”

“…To our knowledge, at least six studies using underwater photogrammetry for coral reef monitoring have been published. The protocols mainly differ in the area sampled for analysis, ranging from 60 to 1,655 m 2 , but are similar in image acquisition procedure, cameras utilized, image processing algorithms, fieldwork environmental conditions during data collection (clear water, shallow sites), use of internal control points, and the colony data processing (Palma et al, 2017(Palma et al, , 2019Lechene et al, 2019;Hernández-Landa et al, 2020;Couch et al, 2021;Urbina-Barreto et al, 2021). UWP relies on the surface analysis of the benthic substrate, and obtained metrics seem to be biased by the area considered.…”

Diver-generated photomosaics as a tool for monitoring temperate rocky reef ecosystems

“…Each sparse cloud was then scaled using image geospatial reference information, checked using the photogrammetric scale bars placed along the transect line and refined using between marker distances. Additionally, the total processing time is dependent on camera image resolution, the number of images being processed and computing power available for use (Bayley and Mogg, 2020;Couch et al, 2021).…”

A National Status Report on United States Coral Reefs Based on 2012–2018 Data From National Oceanic and Atmospheric Administration’s National Coral Reef Monitoring Program

“…These data are a valuable scientific resource, having been broadly used by NOAA, other United States federal agencies, State and Territorial partners, and key stakeholders for a variety of purposes. These include national stock assessments (e.g., Nadon, 2017Nadon, , 2019, endangered species reviews (e.g., Brainard et al, 2011), regional and multinational coral reef assessments (e.g., Brainard et al, 2008Brainard et al, , 2012Brainard et al, , 2019, and peer-reviewed papers geared toward the scientific community (e.g., Enochs et al, 2019;Allen et al, 2021;Couch et al, 2021). The data have also been used to produce technical documents and summary reports (e.g., Boyle et al, 2017;NOAA, 2018;McCoy et al, 2019) aimed at the coral reef management community and other coral reef stakeholder groups to inform resource management decision making, better tailor education and outreach programs at the jurisdiction level, and to evaluate the efficacy of existing programs aimed at reducing coral reef threats.…”