Structure-from-motion reveals coral growth is influenced by colony size and wave energy on the reef slope at Ningaloo Reef, Western Australia

Abstract

A variety of techniques for measuring coral growth, a fundamental biological trait, have been used to improve our understanding of coral demographics. Remaining gaps and biases in this information, in combination with technological advances, led to the present study. We assessed the capacity of structure-from-motion (SfM) technology to build image mosaics (orthophotos) of ~6 × 8 m reef plots from which individual coral colonies could be identified in repeated annual surveys and colony planar area measured. We monitored Acropora, Pocilloporidae (specifically from the genera Stylophora and Seriatopora) and Platygyra colonies over 2 years. We compared growth measurements, as change in radius, from SfM orthophotos to the more traditional method of tagging and photographing individual coral colonies. We conclude that planar coral growth can be measured from SfM orthophotos, with clear advantages over tagging including large sample sizes, and speed and ease of surveying, albeit with some bias in the SfM method (growth estimates were 15% greater (~0.5 cm/year) from standard photos than from the SfM orthophotos). We documented mortality and growth rates in a high wave energy environment on the reef slope at Ningaloo Reef, Western Australia, where there were no existing coral growth or mortality estimates. The mean change in radial extension of Acropora colonies with no mortality was 2.92 ± 0.06 (SE, n = 572) cm year−1 across the pooled data set of standard and orthophotos, substantially lower than previous estimates on similar species from the region. The growth rate of Pocilloporidae was 3.54 ± 0.52 (SE, n = 12) cm year−1 for colonies with no mortality. Robust planar growth estimates were not obtained for Platygyra due to its morphology, but we report high survivorship of the colonies: 94% with no mortality as compared with 79% for Acropora and only 53% for Pocilloporidae. High sample sizes from the novel SfM methodology allowed us to demonstrate that Acropora growth rate was inversely linked to wave energy and coral size.