The world needs coral reefs, and coral reefs need sharks
By Dr. Andy Cornish, Leader of Sharks: Restoring the Balance, WWF’s global shark and ray conservation programme
The value of coral reefs to sharks has long been known. The complex structures that corals form shelter a huge variety of species, creating oases of ocean biodiversity and providing plenty of food for reef sharks. Shallow areas with few predators serve as shark nurseries. But what do sharks bring to the relationship? Recent science is finally shedding light on the subject and indicates that protecting disappearing reef sharks is likely to have considerable benefits for building reef resilience.
My first encounter with reef sharks was when learning to dive in Belize and Honduras in the early ’90s. I was backpacking with a friend before returning to Hong Kong to start a Ph.D. on reef fishes. What I saw in the Caribbean made the complete absence of sharks in Hong Kong and scarcity on otherwise spectacular coral reefs in the Philippines more noticeable in years to follow. I would inquire about enticingly named dive sites such as Shark Corner, only to be told that sharks were very rarely seen there now, if at all.
Anecdotal observations such as mine were reinforced by the literature later on, as scientists started paying more attention to sharks, contrasting their abundance and diversity on heavily and lightly fished reefs. It became clear that even light levels of fishing readily depleted the largest fish such as sharks and groupers — both higher level predators that can have slow reproductive rates.
The scale of ongoing declines of even some of the most ubiquitous reef sharks was brought sharply into focus with the release of the long awaited Global FinPrint project findings in 2020 (MacNeil et al 2020). FinPrint was the first to use the same methodology and analysis across many countries, deploying underwater video cameras on coral reefs with bait to draw the sharks in.
The results were shocking, with 19% of 370 reefs in 58 countries having no sharks at all, and 35 out of 58 nations surveyed having half as many sharks as predicted. Close to zero sharks were detected on reefs in six nations: the Dominican Republic, the French West Indies, Kenya, Vietnam, the Windward Dutch Antilles, and Qatar (only three sharks were seen during 800+ hours of surveys). What wasn’t a surprise was the main cause: overfishing and the use of fishing gear such as longlines and gillnets, which catch a wide range of species very efficiently.
Restoring depleted and declining reef shark populations will require that fishing pressure be reduced, and off limits in some areas or in some seasons. With care, this should benefit other large fish like the groupers and large jack, and since these are important food fishes, this would benefit coastal communities too.
Putting these findings into context requires understanding what should be there — the natural abundance of sharks on coral reefs. It turns out that this is higher than you might imagine, and certainly way higher than my earliest dives 30 years ago would suggest. Historical data on sharks on coral reefs prior to WWII is practically non-existent, so scientists had to look at those most undisturbed by fishing instead.
One of the largest such studies, covering nearly 40 islands and atolls in the Pacific, found that reef shark biomass was four times greater on remote reefs than in the lightly populated areas in the Mariana Islands, and an incredible 50 times greater in remote reef areas than near populated areas in the Hawaiian archipelago! Large predatory fishes such as sharks and jacks were conspicuous and made up a substantial portion of fish biomass at many of the remote reefs but were rarely encountered around populated islands (Williams et al. 2011).
While some studies have reported very high numbers and biomasses of sharks in certain places, such as when grey reef sharks gather in their hundreds to feed on spawning groupers (Mourier 2016), these are unlikely to be representative of entire reef systems. More realistic estimates come from a long-term study in the Great Barrier Reef that found reef shark biomass in no-entry reserves increased and then plateaued after 30 years of protection at around six sharks (or 140 kg of biomass) per hectare (Frisch and Rizzari 2019).
The initial impacts of fishing on pristine coral reefs are that reef sharks, snappers, grouper and the really large jacks tend to be the first species to be fished out. And the consequences of their absence are slowly becoming clearer. Reef sharks can influence prey behaviour merely through their presence. Grazing fishes have been shown to dramatically reduce consumption of seaweed when reef sharks are present, with potential flow-on effects that we don’t yet understand (Rizzari et al. 2014). Declines in sharks can affect the behaviour of stingrays and increase their local numbers, but again, the ecological effects of these changes are as yet unknown (Sherman et al. 2020).
Meanwhile, coral reefs are undergoing their own crisis, but one caused mainly by climate change. Ocean warming and acidification are resulting in widespread coral loss, and a staggering 50% have been lost since the 1980s. If the average global temperature rise is limited to 1.5°C, 70–90% of tropical coral reefs will be lost (Hoegh-Guldberg, OD et al. 2018) and as we have seen from the recent COP26 climate negotiations in Glasgow, higher temperature rises are likely.
Efforts to minimize the threats to coral reefs, on which hundreds of millions of people depend, are rightly focused on reducing atmospheric greenhouse gases. But what if protecting sharks helped build the resilience of coral reefs?
While we don’t understand the full range of ecological roles that reef sharks play, they and coral reefs have co-existed for millions of years and it is highly likely that there are mutual benefits yet to be discovered — especially because shark numbers were so much higher in the past. For example, just 10 years ago it was not known that grey reef sharks assist in distributing nutrients around reefs by defecating nitrogen waste from fish they have eaten away from the reef. This nutrient input is important as coral reefs are generally nutrient poor, and thus these “top-ups” contribute to reef health (Williams et al. 2018).
Newly uncovered examples from other habitats exist too. The role of tiger sharks in limiting seagrass grazing by dugongs and turtles has knock-on benefits for the climate: more seagrass means more carbon storage (Burkholder et al. 2013). It’s only recently that we’ve come to understand that some species of pelagic shark likely maintain the ocean nutrient pump by feeding in the deep, and excreting in the shallows (Martin et al. 2021), fueling plankton productivity in these sunlit but nutrient poor waters.
Other large predatory fishes may perform some similar ecological roles to reef sharks on coral reefs. But their numbers are likely shrinking too, leaving the work of maintaining certain ecosystem functions undone. Now we need a “recruitment fair” to get these fish back on the job.
Restoring depleted and declining reef shark populations will require that fishing pressure be reduced, and off limits in some areas or in some seasons. With care, this should benefit other large fish like the groupers and large jack, and since these are important food fishes, this would benefit coastal communities too. For the reasons we already know and those we are yet to discover, efforts to conserve coral reefs and build their resilience should include a focus on restoring and maintaining reef sharks.
References
Burkholder DA, Heithaus MR et al. (2013). Patterns of top‐down control in a seagrass ecosystem: could a roving apex predator induce a behaviour‐mediated trophic cascade? Journal of Animal Ecology 82(6): 1192–1202
Frisch AJ, Ireland M et al. (2016). Reassessing the trophic role of reef sharks as apex predators on coral reefs. Coral Reefs 35(2): 459–472
Frisch AJ and JR Rizzari (2019). Parks for sharks: human exclusion areas outperform no-take marine reserves. Frontiers in Ecology and the Environment 17( 3): 145– 150
Hoegh-Guldberg, OD et al. (2018) Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty
MacNeil MA et al. (2020). Global status and conservation potential of reef sharks. Nature 583: 801–806
Martin AH, Pearson HC, Saba GK and EM Olson (2021). Integral functions of marine vertebrates in the ocean carbon cycle and climate change mitigation. One Earth https://doi.org/10.1016/j.oneear.2021.04.019
Mourier J et al. (2016). Extreme Inverted Trophic Pyramid of Reef Sharks Supported by Spawning Groupers. Current Biology https://doi.org/10.1016/j.cub.2016.05.058
Rizzari JR, Frisch AJ, Hoey AS, and MI McCormick (2014). Not worth the risk: apex predators suppress herbivory on coral reefs. Oikos 123(7): 829–836.
Sherman CS et al. (2020). When sharks are away, rays will play: effects of top predator removal in coral reef ecosystems. Marine Ecology Progress Series 641: 145–157
Williams I, Richards BL, Sandin S and JK Baum (2011). Differences in Reef Fish Assemblages between Populated and Remote Reefs Spanning Multiple Archipelagos Across the Central and Western Pacific. Journal of Marine Sciences. https://doi.org/10.1155/2011/826234
Williams JJ et al. 2018. Mobile marine predators: an understudied source of nutrients to coral reefs in an unfished atoll. Proceedings of the Royal Society B. https://doi.org/10.1098/rspb.2017.2456
