Tom Arnbom, from the Swedish branch of WWF, tells Radio Sweden he was shocked by the numbers and the extent of the problem.
"It's not just in the Baltic Sea or off the coast of China, it's happening everywhere," he says. "And it's not only fish."
The report from the conservation group, made in conjunction with the Zoological Society of London, said on Wednesday that populations of some commercial fish stocks had fallen by almost 75 percent.
The study said the world's fishing fleets were too big and supported by subsidies. Nevertheless, Arnbom says 3 billion people still depend on protein for the sea.
"And if we can't supply that, what's going to happen is that they're going to have to eat something else and it's pretty hard to find that on shore," he says, "So I think that's what I'm most worried about, the long term. Because if we did this in 40 years time what is going to happen in the next 40 years?"
Arnbom says one solution is to have more protected areas in the ocean. Only 3 percent of the world's oceans are currently protected. Arnbom says he would like to see 20 percent.