While the past century has seen large predatory fish, such as cod and tuna, plummet by two-thirds, small prey fish, such as sardines and anchovies, have more than doubled in number, the research shows.

Although a spike in tiny fish might seem good for fisheries, "only some of these species can be exploited," noted Christensen, of the University of British Columbia in Vancouver. (See pictures of overfishing in National Geographic magazine.)

"In many cases, these are small species there is very little interest in, except maybe for fish feed or fish oil."

Christensen and his colleagues analyzed more than 200 models of marine ecosystems from around the world to estimate fish