A functional reef is more than just coral—it is the entire network of micro and macro flora and fauna living in inter-dependent synergy. Apex predators are essential to healthy reefs, but their role is often overlooked and over half are now
endangered. Is this yet another sign of failing reefs?
Coral reefs are threatened by ocean warming, acidification, and pollution. They are more than just coral structures; every organism within the reef ecosystem plays a role and must be considered in conservation efforts if functional reef
systems are to be maintained.
Despite public campaigns advocating for steps in the right direction, there are still many holes in the net of knowledge and understanding surrounding reef protection and restoration, which pose a barrier to the effective conservation of
these ecosystems.
Coral reefs are found globally, with shallow tropical reefs spanning the equatorial zone and reaching into the Mediterranean, and cold-water reefs fringing deep-sea cliffs of temperate regions, such as the UK and Canada. | Yang
Wewe / Unsplash
Healthy fish populations are vital to reefs, as corals rely on a tight nutrient cycle, kept in balance by fish nutrient cycling. Nutrients are locked in fish body tissue and the excretion of nitrogen and phosphorus through fish gills and
urine. In the correct amounts this is essential to coral growth. Its health is dependent on apex reef predators, such as groupers, snappers, sharks, rays and mammals like monk seals.
However, the role of such species on reefs is often underappreciated, with the effects of their loss only felt once the ecosystem begins to break down. Recent research has found that over 50% of species classified as apex predators on reefs
are now endangered—with shark and ray species of particular concern.
The largest threat to reef sharks and rays is overfishing. This includes direct targeting for market trade such as for shark-fin soup, a highly revered delicacy in southeast Asia, and also accidental bycatch, which is often fatal to the
shark or ray involved.
‘Recent research has found that over 50% of species classified as apex predators on reefs are now endangered.’
Such pressures are exacerbated by the effects of climate change and habitat degradation through anthropogenic pollution and disturbance, all of which negatively affect the survival and reproduction of these species.
Over 94 species of reef shark and ray have been classified as declining in population, highlighting an urgent area of conservation need. As large-bodied carnivores, sharks and rays are highly mobile and are involved in many reef processes.
For example, grey reef sharks feed in the open ocean and are important nutrient transporters from pelagic zones to shallow reefs. Manta rays also feed on zooplankton in the pelagic zone and then deliver these nutrients upon visiting the
reef during the day. Stingrays bury themselves in the sand, initiating sediment turnover and aeration of the seabed, and so on.
Reefs are more than just their coral structures. Every organism within the reef ecosystem plays a role and must be considered in conservation efforts if functional reef systems are to be maintained. | Michael Worden / Unsplash
Smaller shark species, such as black tip, white tip and leopard sharks hunt fish directly on the reef, controlling the population sizes of lower trophic levels and securing adequate nutrient cycling levels. Baited remote underwater video
surveys (BRUVS) carried out across 371 reefs revealed that sharks are functionally extinct at 20% of sites, meaning the role of these apex predators is not being fulfilled.
Without these services, reef systems face a trophic cascade—a series of events causing disruption at each level of the food chain—ultimately causing an imbalance in predator-prey population dynamics and destabilisation of the ecosystem.
This can look like biodiversity loss, algal overgrowth, invasion or takeover by competitive species, and coral death. Consequently, this leads to reduced food stability and loss of livelihoods for communities who rely on coral reefs, a drop
in reef-based tourism revenue, reduced coastal protection and on a larger scale, reduction and loss of globally important fisheries.
Without effective conservation of reef apex predators, coral restoration efforts may be ineffective in the long-term. Only a few policies have been implemented to protect sharks and rays, and the increase in shark finning due to rising
demand is unsustainable. The threat is highest in countries where fishing pressure is high and marine policy is weak.
‘Without apex predators like sharks, reefs face a trophic cascade of disruption at each level of the food chain.’
The sharks and rays of concern are mainly found across countries with high numbers of small coastal communities, unregistered boats, and floating markets dependent on reef fish stocks, making reinforcement of marine policies difficult.
However, without urgent action to reverse population declines, sharks and rays may face local extinctions, with dire consequences for the local communities.
Stricter control and management of fisheries at all scales is required as a starting point for shark and ray conservation. This must be combined with effective marine protected areas (MPAs), in which policy and regulations strictly comply
with no exceptions. This requires an increase in monitoring and policing of protected zones, to inhibit illegal fishing practices and damaging activities such as trawling of the seabed.
International trade regulations must also be addressed to impede the sale of shark and ray products, although their prized value in traditional medicine and cultural rituals poses a challenge.
A cleaner wrasse attends to a foxface rabbitfish. Ballan wrasse and gobies act as cleaners, removing parasites from larger fish species and turtles, helping the overall health of the reef. | David Clode / Unsplash
Improving education about coral reef health and sustainable fishing practices at the local level is equally important, and diversifying the income of local communities can help reduce pressure and reliance on reefs as the sole source of
livelihood.
The diversity of corals provides a nursery habitat for reef fish and crustaceans, which in turn creates a feeding ground for larger fish and marine fauna. While species vary across reef type and location, the major roles in each are
fulfilled by similar animals.
Overall, the future of coral reefs and their apex predators depends on the collaboration of countries across the globe to make the right decisions and take action.
Featured Image: Francisco Jesus Navarro Hernandez | Unsplash
Sherman CS, Simpfendorfer CA, Pacoureau N, et al. (2023) Half a century of rising extinction risk of coral reef sharks and rays. Nature Communications. Volume 14, issue 1, article
number 15.
