Mesophotic Coral Ecosystems

6.2. Fisheries

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;

(a)

(b)

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).

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