Four decades of reef observations illuminate deep‐water grouper hotspots

Abstract

Fish often aggregate to spawn, feed, rest, or avoid predation. Direct observations of very high counts of large‐bodied grouper on deep shipwrecks, however, do not fit into typical descriptions of spawning‐, resource‐, or predation‐driven aggregations. To investigate whether these observations are rare or part of an underlying pattern, we synthesized four decades (1979–2019) of direct observations of groupers on deep‐water (50–300 m) habitats along the southeastern United States (Cape Hatteras, NC to Cape Canaveral, FL). The direct observations, which included 439 remotely operated vehicle transects, 235 human‐occupied vehicle transects, and 881 hook‐and‐line drops, revealed six hotspots of deep‐water groupers on three shipwrecks, two artificial reefs, and one boulder field. Grouper counts at these hotspots (0.10–5.40 grouper per linear m surveyed) exceeded counts of grouper outside of hotspots (<0.01–0.02 grouper per linear m surveyed) by multiple orders of magnitude. Commonalities among the sites with grouper hotspots included that all are relatively isolated structures surrounded by unconsolidated sediments and located in shelf‐edge to upper‐slope depths. Thus, it appears that these isolated habitats, despite their small spatial footprint, represent a disproportionate abundance of deep‐water groupers. Future research efforts should determine how groupers derive sufficient resources from, and thus co‐occur on, these small habitats and how these aggregations relate to the large‐scale dynamics of these populations.

Quantifying spatial extents of artificial versus natural reefs in the seascape

“…Despite their relatively small footprint (<0.01% of seafloor) in the SEUS, these artificial reefs play key ecological roles within the seascape. Within the SEUS region, for example, artificial reefs have been shown to host high abundances of tropical and subtropical reef fish at their poleward climate range edges (Paxton et al, 2019b) and high densities of large predators (Paxton et al, 2020), potentially provide stepping stones or connectivity corridors for large predator movement (Paxton et al, 2019a), and form hotspots for economically valuable fish species (Paxton et al, 2021). The fish communities on artificial reefs differ from those of nearby natural reefs (Paxton et al, 2017;Rosemond et al, 2018;Lemoine et al, 2019) and this translates to differences in species-specific feeding ecology (Lindquist et al, 1994;Pike and Lindquist, 1994).…”

Spatial extent and isolation of marine artificial structures mediate fish density

“…Spatial arrangement of structures can also influence marine life. For example, isolated artificial structures tend to host fish communities characterized by higher abundance and richness (Jordan et al, 2005;Paxton et al, 2021). These characteristics can also affect population level values, such as mortality rates, which were found to be densitydependent around isolated structures, but higher and density independent on structures with less separation from one another (Overholtzer-McLeod, 2004).…”

Comprehensive spatial distribution of tropical fish assemblages from multifrequency acoustics and video fulfils the island mass effect framework

“…The mesophotic zone usually encompasses the shelf-break, a transition area from shelf to ocean characterised by a rapid change in the topography with a steep slope. The stiffness of the slope is associated with turbulent mixing enhancing primary productivity and therefore attracting prey and predators 18 – 20 . It concentrates diverse fishing resources over a relatively narrow area, sustaining important multispecific reef fisheries 21 – 24 .…”