Shark fishing damages coral reefs

Bleaching, pollution, disease outbreaks, tropical storms — the list of threats to coral reefs never seems to end. Now scientists have found that shark fishing in reef areas can affect animals lower on the food chain, leaving corals more vulnerable to stressful events.

Researchers already suspected that removing predators could disturb reef ecosystems. But “[t]he impact of the loss of sharks on coral reefs is not clear,” the authors write in PLOS ONE. Previous studies on the relationship between sharks and other reef species were often done over a short period of time, and this team wanted to tackle the problem at a bigger scale.

So the researchers analyzed data on two sets of reefs near Australia taken from 1994 to 2008. One site, Rowley Shoals, was protected from fishing. At the other site, Scott Reefs, fishermen from Indonesia often hunted for highly-valued species such as sharks and sea cucumbers. The reefs had gone through tough times: a massive cyclone hit Rowley Shoals and a bleaching event decimated Scott Reefs in the 1990s.

Not surprisingly, the density of reef sharks was about three times higher at Rowley Shoals. But the team found differences in other species as well. Scott Reefs had more carnivorous fish, such as snappers, and — after the major disturbances in the 1990s — fewer herbivorous fish, such as parrotfish.

The removal of sharks at Scott Reefs probably allowed smaller carnivores to multiply, which in turn suppressed the number of herbivores, the team says. That’s not good for the reef, because herbivorous fish nibble on algae. Without that constant grazing, algae can smother corals.

Since reefs are especially prone to algal booms after disturbances, “top-order predators may have a role in determining the rate of recovery of reefs from these events,” the authors write. “Healthy populations of reef sharks should be a key target of management strategies that seek to ensure the future resilience of coral reef ecosystems.” Roberta Kwok | 19 September 2013

Source: Ruppert, J.L.W. et al. 2013. Caught in the middle: Combined impacts of shark removal and coral loss on the fish communities of coral reefs. PLOS ONE doi: 10.1371/journal.pone.0074648.

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