The growing demand for shark fin as an ingredient in Chinese cuisine has caused an explosion in the number of shark fisheries in recent decades. But sharks are important members of ocean food chains, and removing them can have unintended consequences. Our new study, conducted off the coast of northwest Australia, shows that killing sharks isn’t just bad for sharks; it can also harm coral reefs.
What do sharks have to do with coral?
Sharks are apex predators - they live at the top of the food chain. They grow slowly, mature late and have relatively low rates of reproduction. This means that their populations have little resilience to harvest and as a result, over-fishing of sharks has now become a worldwide problem.
Nowhere is immune to this phenomenon, as rising prices for shark fin drive fishermen to search every corner of the oceans to harvest sharks. Coral reefs, once renowned for their abundance of sharks, have been targeted by both legal and illegal fishermen so that today even areas as large and as well-managed as the Great Barrier Reef show alarming signs of diminishing shark populations.
While we recognise this loss is occurring, we still have very little idea of what effect the removal of sharks has on coral reef ecosystems.
This is because in most places, fishing is just one of many processes such as coral bleaching, cyclone damage, attacks by crown of thorns starfish, pollution and eutrophication that can occur simultaneously, all of which alter the structure of the reef in fundamental ways. Disentangling the effects of the loss of sharks from these other influences can be a daunting task.
But a unique combination of circumstances now allows us to test the impact of sharks on coral reefs, on offshore atolls in the remote north-west of Western Australia. Our results are published in open journal PLOS ONE today.
For hundreds of years, fishermen from Indonesia travelled south into what are now Australian waters to fish for sharks. This long-standing tradition was recognised when our Exclusive Economic Zone was established; some reefs were set aside so that these fishermen could continue their harvest using traditional methods.
Although traditional, the methods used by Indonesian fishermen are still highly effective at removing reef sharks. Nearby are pristine reefs that are completely protected from fishing. Our decade-long monitoring programs at both these fished and unfished reefs allowed us to compare what happens to reefs with, and without sharks.
The difference between the two is striking. On reefs without sharks, smaller predators (known as “mesopredators”) such as snappers and emperors were many times more abundant.
This phenomenon is called “mesopredator release” in ecology. It’s common wherever top-level predators are removed from food chains both in the ocean and on land. For fishers, having more fish like snappers and emperors might seem like a good thing, but unfortunately the effects of the loss of sharks did not stop at that level in the food chain.
In contrast to the smaller predatory fish, herbivorous or algae-eating fishes (parrotfishes, rabbitfishes and the like) were less abundant on fished reefs. Herbivorous fish are vitally important to coral reefs because they eat algae that otherwise overwhelms young corals, particularly as they recover from disturbances such as cyclones and bleaching.
Given the predicted future of coral reefs under climate change, with more bleaching and cyclones at greater intensity, anything that potentially weakens the ability of reefs to recover is worrying.
We have limited ability to alter the trajectory of our warming climate. At least some of the challenges facing coral reefs are now locked in. But this is not necessarily the case with the loss of reef sharks.
Tracking studies show that in many cases individual reef sharks are closely attached to certain coral reefs, so even relatively small marine protected areas could be an effective way to protect these top-level predators.
Ultimately, this could mean that coral reefs are better able to recover from the serious disturbances they will face in the future.