The world needs coral reefs, but what do the reefs need?
By Dr. Andy Cornish
21 March 2022
The world needs coral reefs, but what do the reefs need?
Most of us would have known that coral reefs are valuable to marine life—they provide shelter for a huge variety of species, create oases of ocean biodiversity and provide plenty of food for marine life. The shallow areas with few predators also serve as shark nurseries, where baby sharks grow!
Sharks depend on coral reefs, 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 effects on the resilience of reefs.
Today's story is a personal experience from Dr. Andy Cornish—Leader of Sharks: Restoring the Balance, WWF’s global shark and ray conservation programme—who's had years of encounters with sharks!
My first encounter with reef sharks
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 the 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.
The status of reef sharks
The scale of ongoing declines was brought sharply into focus with the release of the Global FinPrint project findings in 2020. 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.
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.
Putting these findings into context requires understanding what should be there—the natural abundance of sharks on coral reefs. It turns out that the number of sharks expected is higher than you might imagine, and certainly way higher than my earliest dives 30 years ago would suggest!
One of the largest such studies, covering nearly 40 islands and atolls in the Pacific, showed that large predatory fishes such as sharks and jacks were easily found and made up a substantial portion of fish biomass at many of the remote reefs. However, they were rarely encountered around populated islands.
The potential impact of their decline
The initial impacts of fishing on pristine coral reefs are that reef sharks and other large fishes 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 their consumption of seaweed when reef sharks are present, with potential flow-on effects that we don’t yet understand.
- 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.
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, and as we have seen from the recent COP26 climate negotiations in Glasgow, higher temperature rises are likely.
Efforts to minimise 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, sharks 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 excreting 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.
- 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.
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.
Protecting our coral reefs and sharks
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!
You can now support WWF’s efforts to establish Marine Protected Areas (MPAs), increase research and monitoring efforts, empower local communities, and work with businesses in Singapore to implement a sustainable seafood supply chain. Become a Shark Protector today to join us in the fight to turn things around for sharks!
This article is adapted from an article written by Dr Andy Cornish. For the full story, please visit WWF Oceans
