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Shark Overfishing Harms Coral Reefs: Food Chain Impacts

First Posted: Sep 23, 2013 01:34 PM EDT

Sharks may get a bad reputation in movies, but they contribute quite a bit to the environment. Unfortunately, though, these seafaring carnivores are declining as the practice of shark finning continues. Now, scientists have found that a decline in shark populations is detrimental to coral reefs.

Both coral reefs and sharks are facing numerous impacts. Coral reefs, for example, are experiencing pressures from direct human activity, such as overfishing, as well as from climate change. Sharks, in contrast, are being captured as by-catch as fishermen seek other species, or are being targeted directly for their fins.

In order to see exactly how a lack of sharks could be impacting reefs, the scientists examined locations off of the coast of northwest Australia. There, Indonesian fishermen target sharks, a practice stretching back several centuries.

"The reefs provided us with a unique opportunity to isolate the impact of overfishing of sharks on reef resilience, and assess that impact in the broader context of climate change pressures threatening coral reefs," said Jonathan Ruppert, one of the researchers, in a news release. "Shark fishing appears to have quite dramatic effects on coral reef ecosystems."

So how do sharks affect coral reefs? Sharks eat fish, which impacts the structure of the food chain on reefs. Without sharks, the balance of this food chain shifts and can alter the entire ecosystem.

"Where shark numbers are reduced due to commercial fishing, there is also a decrease in the herbivorous fishes which play a key role in promoting reef health," said Ruppert in a news release.

In fact, the scientists saw increasing numbers of mid-level predators, such as snappers. In contrast, they saw a reduction in herbivorous fishes, such as parrotfishes. These creatures in particular are important to reefs; they eat the algae that otherwise would overwhelm young corals on reefs that are recovering from natural disturbances, such as hurricanes or bleaching events.

 "Given that sharks are in decline on reefs worldwide, largely due to the shark fin trade, this information may prove integral to restoration and conservation efforts," said Ruppert.

The findings are published in the journal PLOS One.

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