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.
Highlights
The Mediterranean Basin is described as a hotspot of diversity for its various fish species and its specific marine ecosystems (Bianchi and Morri, 2000; Cuttelod et al, 2009)
The two methods obtained an equivalent value of cumulative mean rugosity
Rugosity Estimation and Methods Efficiency Habitat type had a significant effect on rugosity: regardless of site and method used, block field habitat consistently exhibited higher rugosity values than continuous rock habitat
Summary
The Mediterranean Basin is described as a hotspot of diversity for its various fish species and its specific marine ecosystems (Bianchi and Morri, 2000; Cuttelod et al, 2009). The infralittoral (i.e., subtidal) rocky reef provides various habitats for different species such as teleost fishes. Climate change and the transformation of habitat due to coastal urbanization are anthropogenic pressures that can impact the 3D structure of these rocky habitats (Thiriet et al, 2014) It may affect endemic species (Airoldi et al, 2008; Coll et al, 2012). The first design evaluated i) the mean rugosity and ii) the efficiency of each method, while the second design was used to study their repeatability. The minimum sampling effort was reached when the cumulative mean rugosity was stabilized This response variable was studied at one site and in one habitat (at Jarre-Jarron, Infralittoral Rock with Photophilic Algae (IRPA): block facies). The responses of mean rugosity and efficiency (time in seconds) were tested as a function of two fixed factors
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