Reef accessibility impairs the protection of sharks

Abstract

Reef sharks are declining world‐wide under ever‐increasing fishing pressure, with potential consequences on ecosystem functioning. Marine protected areas (MPAs) are currently one of the management tools used to counteract the pervasive impacts of fishing. However, MPAs in which reef sharks are abundant tend to be located in remote and underexploited areas, preventing a fair assessment of management effectiveness beyond remoteness from human activities. Here, we determine the conditions under which MPAs can effectively protect sharks along a wide gradient of reef accessibility, from the vicinity of a regional capital towards remote areas, using 385 records from baited remote underwater video systems and 2,790 underwater visual censuses performed in areas open to fishing and inside 15 MPAs across New Caledonia (South‐Western Pacific). We show that even one of the world's oldest (43 years), largest (172 km2) and most restrictive (no‐entry) MPA (Merlet reserve) on coral reefs has between 17.3% and 45.3% fewer shark species and between 37.2% and 79.8% fewer shark abundance than remote areas in a context where sharks are not historically exploited. On coral reefs situated at less than 1 hr of travel time from humans, shark populations are so low in abundance (less than 0.05 individuals per 1,000 m2) that their functional roles are severely limited. Synthesis and applications. Remote areas are the last sanctuaries for reef sharks, providing a new baseline from which to evaluate human impacts on the species. However, there is no equivalent close to human activities even in large, old and strongly restrictive marine protected areas. As such sharks deserve strong protection efforts. The large, no‐entry marine protected areas, close to humans, offer limited benefits for reef shark populations, but provide more realistic conservation targets for managers of human‐dominated reefs. The exclusion of human activities on a sufficiently large area is key to protect reef shark populations. However, this strategy remains difficult to apply in many countries critically depending on reef resources for food security or livelihood.

Global baselines and benchmarks for fish biomass: comparing remote reefs and fisheries closures

“…Large and migratory sharks are examples, but there may be other taxa where movements and spatial needs are less ob vious and poorly studied. Sharks, for example, respond to no-entry and not just no-take management (Claydon 2004, Robbins et al 2006, McCook et al 2010, Juhel et al 2018. Other large-space requiring bony fish species may also be influential in estimating maximum biomass.…”

“…For example, a large survey of reef fish in fisheries closures found that many large top-level predators were virtually absent or too uncommon to sample accurately when closures were located close to fisheries markets (Cinner et al 2018). Yet other studies have shown that the combination of protection from fishing and isolation from human influ-ences can support high biomass and large roaming predators such as sharks (Stevenson et al 2007, McCauley et al 2012, Edgar et al 2014, Bradley et al 2017, Juhel et al 2018.…”

Environmental DNA illuminates the dark diversity of sharks

“…They often have a high intrinsic vulnerability to fishing due to slow population growth ( 9 ), and with shark products such as dried fins reaching high commercial value (up to $1697 kg −1 ), they have a high exposure to international trade ( 10 ). Throughout the Pacific, the density of reef sharks has declined to 3 to 10% of prehuman levels ( 11 ), and even the most well-managed marine protected areas appear inadequate in maintaining healthy shark populations ( 8 , 12 ).…”

Community biomass and life history benchmarks for coral reef fisheries

“…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., ; Graham & McClanahan, ). 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., ; Juhel et al., ; McCauley et al., ; Nadon et al., ). Populations of these taxa decline with very low levels of fishing effort, and most closures in fished landscapes do not sustain large‐bodied and space requiring species (Graham et al., ; McClanahan, Graham, et al, ).…”

“…While the practical need is great, the high diversity and spatial heterogeneity of reef fishes may preclude finding useful sustainability benchmarks. For example, the maximum biomass of fish has been reported over large ranges and often associated with isolation from human influences and the existence of large roaming predators, such as sharks (Bradley et al., ; Juhel et al., ; McCauley et al., ; Stevenson et al., ). Further, large differences in biomass exist between remote wilderness and unfished national parks, indicating that human disturbance is widespread and not just limited to local fishing impacts (Graham & McClanahan, ).…”