Mesophotic Coral Ecosystems

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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 64
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
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 ). 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).
5.2. Essential habitat