Four decades of reef observations illuminate deep‐water grouper hotspots

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.