Variation in abundance, diversity and composition of coral reef fishes with increasing depth at a submerged shoal in the northern Great Barrier Reef

Abstract

Coral reef fishes often exhibit specific or restricted depth distributions, but the factors (biotic or abiotic) that influence patterns of depth use are largely unknown. Given inherent biological gradients with depth (i.e. light, nutrients, habitat, temperature), it is expected that fishes may exploit certain depths within their environment to seek out more favourable conditions. This study used baited remote underwater video (BRUV) systems to document variation in the taxonomic and functional (trophic and size) structure of a fish assemblage along a shallow to upper-mesophotic depth gradient (13–71 m) at a submerged, offshore shoal in the northern Great Barrier Reef. BRUVs were deployed during two separate time periods (February and August 2017), to separately examine patterns of depth use. Both the relative abundance and diversity of reef fishes declined with depth, and there were pronounced differences in the taxonomic and functional structure of the fish assemblage across the depth gradient. In shallow habitats (< 30 m), the fish assemblage was dominated by herbivores, detritivores, planktivores and sessile invertivores, whereas the fish assemblage in deeper habitats (> 30 m) was dominated by piscivores and mobile invertivores. Depth and habitat type were also strong predictors for important fisheries species such as coral trout (Plectropomus spp.), emperors (Lethrinus spp.) and trevallies (Carangid spp.). We found limited evidence of temporal changes in depth and habitat use by fishes (including fisheries target species), although recorded temperatures were 4 °C higher in February 2017 compared to August 2017.

Re-visiting the paradigm of coral reef depth zonation on contemporary reefs

“…We found that the greatest spatial variance was at the site-scale for primary and secondary consumers, piscivores, and total biomass, indicating that intra-island heterogeneity in habitat availability 78 , local scale hydrodynamics 14 , and local disturbance dynamics 22,68,79 may be predominant mediators of the biomass of those groups 42 . Spatial variance at the site-level was particularly high (64%) for secondary consumers, emphasising trends in location-speci c variability in their biomass-depth relationships described in previous studies 53,54 . Conversely, spatial variance in planktivore biomass was greatest at the ecoregion-scale, pointing towards regional disparities in primary production 39 and the availability of pelagic subsidies as a primary correlate in the distribution of planktivorous sh biomass 33,57 .…”

“…Combined, the results suggest the existence of lower and upper local-scale thresholds in critical reef steepness in mediating delivery of allochthonous subsidies into the shallows and that these effects propagate through to determine the natural carrying capacity of speci c trophic groups of reef shes. This upwelling in uence might also explain the previously documented spatially variable peak in planktivorous and piscivorous shes at depths beyond the 30 m limit of this study 53,67 .…”

“…Fishing reduces the overall standing biomass of reef shes across trophic groups 7 , often with marked losses of piscivores and herbivores 46,[69][70][71] . Our ndings of diminished biomass of primary consumers from shallow depths and piscivores and planktivores between 0-30 m on populated reefs does not exclude the possibility of mesophotic refugia for depth-generalists 46,53,54,72 . It does, though, underscore the vulnerability of herbivorous shes that are largely restricted to shallow reef zones 73 .…”

“…Focussing on the shallowest 0-30 m, we show that secondary consumer biomass increased between 0-15 m then plateaued. This diverse trophic group includes macro-and sessile invertivores and omnivores whose biomass can vary differentially with depth at local scales 53,54 . However, broad energetic pathways are governed by primary productivity, generally with nutrient limitations in the shallows 35 but greater productivity with depth, which at broader scales may cause the more consistent zonation pattern of this group 28,55 .…”

Improved Baited Remote Underwater Video (BRUV) for 24 h Real-Time Monitoring of Pelagic and Demersal Marine Species from the Epipelagic Zone

“…Since the mid-1990s [4], BRUV-based methods have been developed to assess the abundance, diversity, and behavior of different species [5,6] They are a cost-effective and safer alternative to other methods, such as underwater visual surveys (UVS), diver-operated video (DOV), or Remotely Operated Vehicles (ROVs) [4,7,8]. Most studies have focused predominantly on fish populations [4,7,9], but have also been applied to invertebrates [10] and large marine predators, such as bony fishes and elasmobranchs [8,11,12]. Shark stock assessment with BRUVs is effective due to their strong attraction to bait [13].…”