Coexistence of Low Coral Cover and High Fish Biomass at Farquhar Atoll, Seychelles

Abstract

We report a reef ecosystem where corals may have lost their role as major reef engineering species but fish biomass and assemblage structure is comparable to unfished reefs elsewhere around the world. This scenario is based on an extensive assessment of the coral reefs of Farquhar Atoll, the most southern of the Seychelles Islands. Coral cover and overall benthic community condition at Farquhar was poor, likely due to a combination of limited habitat, localized upwelling, past coral bleaching, and cyclones. Farquhar Atoll harbors a relatively intact reef fish assemblage with very large biomass (3.2 t ha−1) reflecting natural ecological processes that are not influenced by fishing or other local anthropogenic factors. The most striking feature of the reef fish assemblage is the dominance by large groupers, snappers, and jacks with large (>1 m) potato cod (Epinephelus tukula) and marbled grouper (E. polyphekadion), commonly observed at many locations. Napoleon wrasse (Cheilinus undulatus) and bumphead parrotfish (Bolbometopon muricatum) are listed as endangered and vulnerable, respectively, but were frequently encountered at Farquhar. The high abundance and large sizes of parrotfishes at Farquhar also appears to regulate macroalgal abundance and enhance the dominance of crustose corallines, which are a necessary condition for maintenance of healthy reef communities. Overall fish biomass and biomass of large predators at Farquhar are substantially higher than other areas within the Seychelles, and are some of the highest recorded in the Indian Ocean. Remote islands like Farquhar Atoll with low human populations and limited fishing pressure offer ideal opportunities for understanding whether reefs can be resilient from global threats if local threats are minimized.

Managing coral reef fish community biomass is a priority for biodiversity conservation in Madagascar

“…Some remote large wilderness areas have higher biomass, > 2000 kg ha −1 , mostly at depth (Graham & McClanahan 2013, Friedlander et al 2014. Nevertheless, the most remote areas studied here, such as the Barren and Mitsio Islands, had variable biomass.…”

“…Fish aggregate in areas of moderate to high coral cover but the implication of these findings is that biomass itself is the cause of the diversity not the coral refuge. In support of this contention, a number of studies have found that fish are more resilient to losses of coral cover than expected from correlation analyses (Gra-ham et al 2008, Friedlander et al 2014, Lamy et al 2015. This provocative finding indicates the difficulties in evaluating the possible causation, interdependencies, time lags, and feedback in complex ecosystems.…”

Community biomass and life history benchmarks for coral reef fisheries

“…Evaluations of fisheries should consider that the benchmark reported here is already biased towards yields and risk because it was established from closures often in heavily fished seascapes. Reported biomass in closures is lower, possibly 2 times lower, than reports in the very few remaining wilderness seascapes of the Indian Ocean (Friedlander et al, 2014;Graham & McClanahan, 2013). These differences are caused by greater numbers of large-bodied and roaming species, including groupers, jacks, snappers, sweetlips and shark populations, in wilderness than closures (Bradley et al, 2017;Juhel et al, 2017;McCauley et al, 2012;Nadon et al, 2012).…”

“…Ecological studies reporting more significant associations may be reporting responses on smaller spatial scales or from reefs where target biomass was reduced by fishing. Studies of the responses of fish to losses of hard coral cover often report reductions in small‐bodied and coral‐dependent species, which do not contribute greatly to total biomass (Friedlander et al., ; Graham et al., ; Lamy, Legendre, Chancerelle, Siu, & Claudet, ; Pratchett et al., ). Some studies indicate that diversity of benthic‐attached reef fish is maximized at modest levels of coral cover of ~25% (Wilson et al., ).…”

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“…However, only a few localities worldwide still 45 maintain large abundances of top predatory fishes due to either being remote and unfished, or 46 having recovered after full protection from fishing (Sandin et al, 2008; Aburto-Oropeza et al, 47 2011;Friedlander et al, 2014a). The small number of scientific 48 studies on relatively pristine ecosystems limits our ability to establish true baselines of sharks 49 and other large predatory fish abundance and this restricts our capacity to determine realistic 50 recovery targets for degraded ecosystems (McClenachan et al, 2012;Sala 2015), thus 51 perpetuating the shifting baseline syndrome (Pauly, 1995;Jackson, 2010).…”