Depth-Dependent Structuring of Reef Fish Assemblages From the Shallows to the Rariphotic Zone

Paris V Stefanoudis,Struan Robertson Smith,Joanna M Pitt,Lucy C Woodall,Todd Kincaid,Alex D Rogers,Gwilym Rowlands,Molly Rivers,Erika Gress,Dominic A Andradi-Brown

doi:10.3389/fmars.2019.00307

Abstract

Shallow coral reef ecosystems worldwide are affected by local and global anthropogenic stressors. Exploring fish assemblages on deeper reefs is therefore important to examine their connectivity, and to help understand the biodiversity, ecology, distinctiveness, evolutionary history and threats in this sparsely studied environment. Conducting visual surveys on the Bermuda slope and a nearby seamount at depths from 15‒300 m, we document decreasing fish biomass and diversity with increasing depth. Fish assemblages were primarily depth-stratified, with distinct suites of species inhabiting shallow (<30 m depth) and upper (60 m) and lower (90 m) mesophotic coral ecosystems, and confirming the presence of a distinct rariphotic (~150‒300 m) assemblage. We also report evidence of anthropogenic pressures throughout our surveyed depths. Our results highlight the novelty of deeper reef fish faunas, therefore suggesting limited applicability of the deep reef refuge hypothesis, and showcase the vulnerability of deep reefs to targeted fishing pressure and invasive species.

Highlights

Around 6,300 fish species are known to occur in coral reef ecosystems worldwide (Parravicini et al, 2013), many of which are commercially exploited
We report that fish biomass and species richness decline with depth, richness first peaked at 30 m, the boundary depth at which shallow and upper mesophotic coral reef fish faunas meet
We provided evidence that depth is a strong predictor of fish assemblage structure with different suites of species occupying shallow, upper and lower mesophotic, and rariphotic coral reef habitats, respectively

Summary

Introduction

Around 6,300 fish species are known to occur in coral reef ecosystems worldwide (Parravicini et al, 2013), many of which are commercially exploited. Several studies have found that lower MCEs (∼60–150 m) support fish assemblages distinct from upper MCEs and shallower habitats (Kahng et al, 2017; Semmler et al, 2017; Sih et al, 2017), suggesting even greater partitioning than previously thought These assemblages are typified by the increased occurrence of endemic or undescribed species with increasing depth (Kane et al, 2014; Fukunaga et al, 2017; Kosaki et al, 2017) and warrant sustainable management and conservation in themselves. Recent research from this deeper reef zone, defined as “rariphotic” (∼150–300 m, sensu Baldwin et al, 2018), provided evidence of distinct fish assemblages that have little overlap with those of MCEs but which are dominated by deep-living representatives of taxonomic groups (i.e., families) usually found in shallow water as opposed to the deep sea (Sih et al, 2017; Baldwin et al, 2018)

Methods

Results

Discussion

Conclusion