A study in Science journal says climate change, fishing, pollution and other human factors have exacted a heavy toll on almost half of the marine waters. Only remote icy areas near the poles are relatively pristine, but they face threats as ice sheets melt, they warn.

The authors say the data is a "wake-up call" to policymakers.

Lead scientist, Dr Benjamin Halpern, of the National Center for Ecological Analysis and Synthesis in Santa Barbara, US, said humans were having a major impact on the oceans and the marine ecosystems within them.

 "In the past, many studies have shown the impact of individual activities," he said. "But here for the first time we have produced a global map of all of these different activities layered on top of each other so that we can get this big picture of the overall impact that humans are having rather than just single impacts."

 Soruce :  BBC Science