As shark populations decline, fish face less pressure from the top of the food chain. As a result, new research shows, fish are getting fatter.

Researchers from the University of Western Australia and the Australian Institute of Marine Science studied fish behavior in Rowley Shoals and Scott Reefs. The former, a marine preserve, hosts healthy shark populations. The latter, an atoll-like reef off the northern coast of Australia, is a popular location for shark fishermen from Indonesia.

The team of scientists observed reef fish spending more time hunting and feeding in the water column near Scott Reefs, where sharks are rare. Spending time in the water column puts fish at risk of ambush, but it's also home to more energy-rich prey.

Fish living among Rowley Shoals were observed mostly hunting and feeding close to the coral reefs, where sharks are less abundant and escape routes are plentiful. Food here, however, is less calorie-dense.

Scotts Reefs fish were 28 percent heavier than those from Rowley Shoals.

"Fear is known to be an important driver of behaviour in animals," marine scientists Shanta Barley said in a news release. "When the risk of being attacked is high, prey eat less, fight less, mate less and, in general, do less. They also spend more time in shelter."

Shark populations around the globe continue to shrink. More than half of all shark and ray species are vulnerable or threatened. Yet, the ecological impacts of their losses aren't well understood.

"These changes may have important implications for coral reefs," said Jessica Meeuwig, a professor at UWA. "Fatter fish produce more offspring than skinnier fish and have better survival rates, so our results suggest that shark overfishing has the potential to transform reefs."

A number of studies have highlighted the importance of large marine predators to ecological balance and ocean health.

Researchers published their latest findings in the journal PLOS ONE.

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