Mesophotic Coral Ecosystems

5.2. Essential habitat

Nassau grouper ( E. striatus ). Depths ranged from 35–40 m on top of the shelf, although yellowfin and Nassau groupers can descend to 60 m during spawning (Nemeth 2005, Kadison et al. 2011). Similar depth ranges were reported for yellowfin, black ( M. bonaci ) and Nassau groupers at sites in the Mona Passage off western Puerto Rico (Schärer et al. 2012, 2014, Tuohy et al. 2015) that have similar depth profiles. The shelf break off the north coast of St. Thomas occurs much deeper (70–80 m) and spawning aggregations of several species have been reported by fishermen, including blackfin snapper ( Lutjanus bucanella ), a species limited to mesophotic and deeper depths (Ojeda- Serrano et al. 2007). Similarly, in the Indo-Pacific, spawning aggregations of the camouflage grouper ( E. polyphekadion ), brown marbled grouper ( E. fuscoguttatus ) and squaretail coral grouper ( Plectroplomus areolatus ) occur typically in shallow depths (Rhodes and Sadovy de Mitcheson 2012), but have been reported to depths of 40m (Rhodes 2012) for the former species and 50 m (Tamelander et al. 2008) for the latter two species. Off the west coast of Florida, the shelf-edge reefs are located at depths greater than 50 m and it is here that gag ( M. microlepis ) and scamp ( M. phenax ) groupers aggregate to spawn (Coleman et al. 1996, Koenig and Coleman 2012). Deeper still at 60–80 m is Pulley Ridge, a mesophotic reef in the Gulf of Mexico, where large red grouper ( E. morio ) spawn. Red groupers are nest builders, and scour out burrows 10 m in diameter that form oases for small reef fish (Reed et al. 2015). Each burrow has a single male or female grouper and multibeam sonar shows that these pits are very evenly spaced, at about 100 m apart. The breeding population within the Pulley Ridge marine protected area may exceed 130,000 burrows, not only providing unique habitat features, but also exporting larvae downstream to shallow reefs, such as the Florida Keys. The occurrence of important transient spawning aggregations within mesophotic depths is probably not uncommon where the appropriate geomorphology exists. However, their distribution and numbers are probably underrepresented; owing to the difficulty in working at mesophotic depths and the general lack of depth information reported in many species accounts (see Chapter 12 in Sadovy de Mitcheson and Colin 2012 or www.scrfa.org). Fish aggregations have historically been overexploited by commercial fishers, even within mesophotic depths (e.g. Olsen and LaPlace 1978), and for many species, mesophotic aggregations are the only known sources of larvae left (Roberts 1996). Some transient aggregations are protected by temporary or permanent no- take restrictions (Nemeth 2005); however, the location and status of the vast majority of mesophotic aggregations are largely unknown and remain unregulated. These remarkable aggregations are a unique ecosystem service provided by MCEs; one that is critical to the continued recruitment of commercially- and ecologically-important fish species (Figure 5.2).

MCEs provide essential habitat for fish and other mobile species to spawn, shelter, feed and/or grow to maturity. Both mesophotic and shallow reefs enhance biodiversity through supporting fish in MCEs with significant connectivity to shallow areas, provide a refuge function from overexploitation that allows species to increase biomass, maintain higher numbers of species and individuals and support key ecological functions (e.g. predation and top-down control of community composition and maintenance of spawning stocks for fish settling in shallow reef ecosystems): all of which enhance overall system stability and resilience. MCEs provide food and shelter for threatened species, such as sharks (Bejarano et al. 2014) and marine turtles (Appeldoorn et al. 2015), and serve as key habitat for a wide variety of fish, particularly large commercially-important snappers and groupers (Brokovich et al. 2008, Bejarano et al. 2014). Many commercially- and ecologically-important fish species have distributions that extend into mesophotic depths throughout the year, and still others are depth specialists found only in the deeper portion of the mesophotic zone (Brokovich et al. 2008, Bejarano et al. 2014). Mesophotic fish are generally easily exploited using traditional fishing gear (Sattar and Adam 2005, Wood et al. 2006), and in some areas, MCEs represent an opportunity for potential fishery expansion; while in others, there has already been a substantial depletion of commercially-important species (see Case Study Box, Chapter 6). MCEs can serve a critical role as a refuge area to protect species overexploited in shallower depths (Bejarano Rodríguez 2013) from fishing. The essential role of MCEs in fish production and the maintenance of biodiversity is further illustrated by the large degree of connectivity between shallow reefs and MCEs. Mesophotic fish enhance this ecological connectivity following one or more strategies, including recruitment and residence across the full depth range, deep recruitment and upward migration, shallow recruitment and offshore migration, and migration to specific transient spawning aggregations (Bejarano Rodríguez 2013). Many large-bodied coral reef fish form transient spawning aggregations on the edge of insular or continental shelves, sometimes at promontories, or along the sides or bottoms of channels. Individual fish may travel tens or even hundreds of kilometres to these aggregation sites (Bolden 2000, Nemeth et al. 2007). Transient spawning aggregation sites are typically at the edge of shelves and thus, depending on the species and local geomorphology, can occur in their entirety or in part within mesophotic depth ranges. For example, along the shelf edge south of St. Thomas, U.S. Virgin Islands, spawning aggregations have been documented for red hind ( Epinephelus guttatus ), yellowfin grouper ( Mycteroperca venenosa ) and

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