The Three-Dimensional Structure of Mediterranean Shallow Rocky Reefs: Use of Photogrammetry-Based Descriptors to Assess Its Influence on Associated Teleost Assemblages

Abstract

In the Mediterranean Sea, shallow rocky reefs and the associated three-dimensional (3D) structure support rich and abundant communities; they are therefore of functional importance, in particular for the renewal of fish stocks. However, these habitats and their functions are likely to be altered by anthropogenic pressures inducing habitat transformations. It is therefore necessary to assess their 3D structure, their transformations and relationship to communities, especially for management and conservation purposes. In this article we aimed (i) to compare two methods that quantify the metrics of the 3D structure (rugosity) of shallow rocky reefs (chain-and-tape method and photogrammetry), and (ii) to quantify the possible links between this habitat structure and the fish assemblages. We found that photogrammetry and the chain-and-tape method yielded a similar estimate of rugosity, but photogrammetry was the most efficient method in terms of measurement quality and time (when considering in-water acquisition). This method also displayed the best repeatability. The 3D habitat descriptors (mean surface rugosity, variation of surface rugosity, and depth) differed significantly between the studied sites and were therefore included as covariables. Total fish abundance and species richness increased with higher mean surface rugosity. In addition, the composition of fish assemblages was significantly influenced by surface rugosity, although this effect was modulated by depth. When focusing on specific taxa, neither density patterns nor size class distributions displayed clear patterns in relation to rugosity metrics. However, this study demonstrated that spatial variability of teleost fish assemblages can be explained by habitat rugosity which probably increases the number of shelters and food resources, and therefore improves chances of survival. In addition, our study has shown that photogrammetry is an appropriate method to assess 3D structure metrics in a temperate rocky reef.

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

“…Importantly, SfM methodolgy conferred the additional advantage of only one tenth of the total data collection time occuring underwater. Monfort et al (2021) discussed similar findings where rugosity measured from SfM derived 3D point clouds was quicker than in situ 'chain and tape' measurements, while Couch et al (2021) found that photomosaics yeilded similar coral colony data to in situ data collection, but reduced field time by 55%. This reduction in field time is consistent with the use of other photographic techniques, such as photoquadrats, over field quadrats (Preskitt et al, 2004).…”

“…These non-rigid taxa will continuously move in relation to water motion which has the potential to hinder the SfM reconstruction process, as has been shown in terrestrial systems when surveying vegetation in windy conditions (Dandois et al, 2015;Fraser and Congalton, 2018). Diverbased SFM photogrammetry work on algal dominated rocky reefs has to date been limited in spatial scale and focussed on measuring a single metric such as reef rugosity (Monfort et al, 2021), substrate composition (Ternon et al, 2022) or quantifying crustose coralline algae cover underneath kelp canopies (Smale et al, 2020). The potential for using SfM photogrammetry for collecting data from temperate rocky reef ecosystems, particularly as an alternative to, or supplement to traditional SCUBA-based methods for benthic monitoring, requires further investigation.…”