If the shark dies, the reef dies
If the large predatory fish disappear, this triggers a chain reaction that benefits smaller predatory fish and crown-of-thorns starfish - but not the coral reef.
Sharks and coral reefs around the world are under pressure: an estimated 100 million sharks are caught and killed every year, while reefs are suffering from climate change and ocean acidification. A study by Mark Meekan from the University of Western Australia in Crawley and his team shows that the disappearance of large predatory fish is exacerbating the situation for reefs regionally: It triggers a chain reaction in the food chain, at the end of which the corals are ultimately eaten and destroyed en masse by crown-of-thorns starfish (Acanthaster planci).
The starfish are a normal component of many reefs in the Indo-Pacific region and feed on stony corals, which they graze on. They themselves are eaten by medium-sized predatory fish such as snappers or bighead snappers, which in turn are prey for larger reef sharks. Ultimately, the populations of some sharks also keep the populations of others in check. In addition, the sharks also influence the behaviour of smaller predatory fish that live near the bottom or corals when sharks are present in the ecosystem. There, snappers and co. increasingly feed on crustaceans, molluscs and starfish.
In many regions, however, the number of sharks has been drastically reduced by fishing or they have disappeared completely. Although this has led to an increase in the number of smaller predatory fish (provided they are not also fished), they have also changed their hunting grounds: they now focus on open water and prey such as smaller fish or squid, which reduces the hunting pressure on the crown-of-thorns starfish, among others. These reproduce more strongly and feed more intensively on the reef.
This is confirmed by the scientists' data: in marine reserves where there is no fishing and top predators are also present, the number of crown-of-thorns starfish is considerably smaller and the reefs are healthier and more resilient. In other parts of the Indo-Pacific, on the other hand, where fishing is intensive and sharks in particular are hunted, there are now regular large-scale proliferations of starfish with serious consequences for the reefs.
In view of the strong pressure that these ecosystems are already under due to climate change or water pollution, Meekan and Co are calling for urgent changes to fishing practices in many parts of the Indo-Pacific. In addition to the establishment of further marine reserves, sharks in particular must be given greater protection in order to preserve their controlling role in the food chain.
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