Populations of marine mammals, birds, reptiles and fish have dropped by about half in the past four decades, with fish critical to human food suffering some of the greatest declines, WWF warned Wednesday.

"In the space of a single generation, human activity has severely damaged the ocean by catching fish faster than they can reproduce while also destroying their nurseries," Marco Lambertini, head of WWF International, said in a statement.

The World Wildlife Fund's latest report, which tracked more than 1,200 species, found a 49 percent decline in marine populations from 1970 to 2012.