Mesophotic Depth Gradients Impact Reef Fish Assemblage Composition and Functional Group Partitioning in the Main Hawaiian Islands

Abstract

Mesophotic coral ecosystem (MCE) research has increased considerably in recent years, as MCEs may provide partial insulation from the effects of climate change, localized anthropogenic stressors, and may dampen fishing pressures for target species depleted in shallower waters. However, few studies have examined coral reef fish assemblages and functional groups across shallow water to mesophotic depth gradients. In the Main Hawaiian Islands, we investigated coral reef fish communities between 0 and 100 m using baited remote underwater stereo-video. While significant community shifts were detected when transitioning from shallow water to mesophotic depths, relative abundance and species richness remained highest between 0 and 30 m. Mobile invertivores and generalist macropiscivores were exceptions, recording higher abundance and richness values in deeper waters. Depth, habitat complexity, and percent cover of unconsolidated sediment and macroalgae were the main reef fish community drivers in multivariate regression and distance-based linear models. Finally, several target species were more abundant and/or larger in deeper waters, suggesting stock assessment and resource management strategies are incomplete without the incorporation of mesophotic portions of stocks.

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

“…Fish abundance changed with depth but without a clear directional pattern, with surveys from deeper depths exhibiting a greater amount of variability. Previous findings from Caribbean (Bejarano et al, 2014;Pinheiro et al, 2016), and Pacific reefs (Thresher and Colin, 1986;Fukunaga et al, 2016;Asher et al, 2017;Sih et al, 2017), largely support a decline in fish abundance with depth, although a recent survey, also from Bermuda, recorded a depth-related increase in fish abundance within 45 m and 80 m (Pinheiro et al, 2016). Interestingly, the depths examined for that study overlap with the 60 m depth band in our study, which exhibited the greatest variability in fish abundance between surveys.…”

“…It is clear that adoption of protocols for standardizing the collection of data is key for the inter-comparability of future reef studies (Woodall et al, 2018). In addition, sitespecific characteristics such as structural habitat complexity (i.e., continuity of reef habitat, topography, availability of hard substratum) have been shown to affect fish community patterns (Brokovich et al, 2008;Garcia-Sais, 2010;Asher et al, 2017) (see also Table 3). Notably, we found benthic community and substratum composition to be most variable between depths of 60-200 m (Figures 6I,J), which overlaps with the studied depths (45-80 m) of the previous survey in Bermuda (Pinheiro et al, 2016).…”

Global community breaks at 60 m on mesophotic coral reefs

“…The changes observed here, using presence/absence data of many diverse taxa, are likely to be indicative of changes in community structure and of changes in community composition, if combined with the observations of changes in abundance with depth (e.g., . Although the generality of the community break at ~ 60 m has been questioned (e.g., Pyle et al, 2016, based on fish assemblages in Hawaii), this evidence is equivocal; many other studies indicate that fish assemblages from Hawaii and elsewhere differ significantly with increasing depth into the mesophotic zone (Asher, Williams, & Harvey, 2017;Baldwin, Tornabene, & Robertson, 2018;Bejarano, Appeldoorn, & Nemeth, 2014;Pinheiro et al, 2016;Rocha et al, 2018). influence the community structure of MCEs (Bridge et al, 2012;Wolanski, Colin, Naithani, Deleersnijder, & Golbuu, 2004).…”

Highly diverse mesophotic reef fish communities in Raja Ampat, West Papua

“…show declines (Asher et al 2017). Given that we detected a significant interaction between 538 depth and habitat complexity when predicting fish biomass (Table 5), it is likely that changes 539 in abundance are also affected by differences in benthic habitats.…”