68 / 100
MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
68
The pressure from fisheries is considered by marine experts
to be the greatest threat to all benthic habitats (Baker and
Harris 2011). Fishing also represents a major threat to MCEs,
both through the perturbation of trophic relationships and
through physical damage to the reef structure. While in
general, many MCEs have not been exploited to the same
extent globally as shallower reefs (due in part to their relative
geographic isolation and morphology, which impedes many
fishing methods), overexploitation of shallower reefs and a
resulting reduction in catches is leading to the expansion of
fishing activities into MCEs.
Commercially-valuable fish species have been identified
withinMCEs, including species of high conservation concern
(such as sharks and groupers). It has been suggested that
following the onset of fishing, most commercially-valuable
fish species suffer large population declines (Bejarano
Rodríguez 2013, Williams et al. 2015). For example, many
large-bodied grouper species form annual spawning
aggregations at the edge of continental shelves, which
— depending on the location — can occur at mesophotic
depths (see Chapter 5.2 Essential habitat). The predictability
in time and space of these grouper spawning aggregations
have made them particularly vulnerable to overexploitation
(De Mitcheson et al. 2008). As a result, many are now
functionally extinct in large parts of their range. To combat
this, Caribbean countries have instigated seasonal fisheries
closures during spawning (Starr et al. 2007). In Cuba, fisheries
studies have indicated that species with greater predictability
in aggregation location and timing show greater declines in
fisheries landings than species with less predictable and less
concentrated aggregations (Claro et al. 2006).
As shallow reefs come under increasing pressure, many MCEs
in areas accessible to artisanal fishing communities are likely
to be fished. These fisheries often involve traditional fishing
techniques, such as hook and line, which allow communities
to catch mesophotic reef fish. For example in the Maldives,
grouper are fished (mostly for export) down to 80 m using
this technique (Sattar and Adam 2005). Similarly in Malaysia,
where fishing is carried out from small boats, artisanal fishers
use mostly hook and line, with a small number using longlines
(Wood et al. 2006). Longlines are either deployed near the
surface to catch pelagic fish such as tuna, or directly onto
mesophotic reef slopes for demersal species, such as sharks
and groupers (Wood et al. 2006). These techniques are fairly
typical of many Southeast Asian mesophotic reef fisheries.
Satellite location systems and depth sounders allow easier
identification of offshore MCEs as potential new fishing
grounds.The arrival of even relatively established technologies
into new areas can open up MCEs for fishing. For example,
spearfishing in the Mariana Islands was traditionally limited
to free diving on shallow reefs but following the introduction
of scuba diving, fishing has expanded into the mesophotic
zone (Myers 1993, Lindfield et al. 2014).
Trawling is a major threat to many mesophotic communities,
with benthic trawl nets designed to be dragged close to the
seafloor. In addition to destroying MCEs through the direct
impact of their large metal balls or plates on the bottom,
trawls often stir up sediment that smothers corals, causing
their death (Erftemeijer et al. 2012).
Documented examples of trawling impacts to MCEs are
limited. Thus, to demonstrate the devastating effects that
bottom trawling can have on MCEs, we must look to the
azooxanthellate ivory tree coral reefs,
Oculina
varicosa
,
found at mesophotic depths off Florida, USA (70–90 m;
6.2.
Fisheries
Figure 6.3.
Deep-water
Oculina
varicosa
off Cape Canaveral (67 m
depth). (a) Historical photo from 1976, and (b) Same site in 2001
reduced to rubble from apparent bottom trawling (photos Reed et
al. 2007, used with permission).
(a)
(b)