There are still plenty of fish in the sea—they're just the little ones, new models suggest.

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.

Overfishing of larger species has led to an explosion of smaller prey fish—what fisheries scientist Villy Christensen calls a "'when the cats are away, the mice will play' effect."

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."

Fish Crashes Due to Overharvesting

Christensen and his colleagues analyzed more than 200 models of marine ecosystems from around the world to estimate fish numbers between 1880 and 2007. They found that 54 percent of the decline in predatory fish populations had occurred in the past 40 years.

"Cod in the North Atlantic is a classic case of a crash," he said. "You had hundreds of years of sustainable fishing of cod—on the order of 200,000 to 300,000 tons caught a season—and then we saw an increase up to 700,000 tons, beyond sustainable levels.

"Now it's been 20 years and we still haven't seen a recovery."

Christensen noted that some might say a two-thirds decline in large fish is exactly what is needed for fishing at maximum sustainable yield.

"That is correct," he noted. However, their estimates are likely conservative, and "we see no indication that the decline has stopped."

Big Fish Decline Creating Imbalance

Additionally, the boom in small fish may just add to a natural imbalance left by the absence of the bigger fish.

"Systems where predators have been decimated tend to be unstable," said Christensen, who presented the unpublished results February 18 at an American Association for the Advancement of Science meeting in Washington, D.C.

"Without predators, there is no one to weed out the sick and weak individuals, and we know from terrestrial systems that this often causes crash-boom situations."

A lack of large predators leads "to less biologically diverse ecosystems [that] become much less resilient to pressures such as pollution, climate change, or changes in [ocean] currents or food supply," said Jacqueline Alder, coordinator of the marine and coastal branch of the U.N. Environment Programme.

"We might need a shift in mindset to think of fish as a scarce resource rather than a global commons," said Alder, who did not take part in the study.

Climate Change May Affect Fish

A key focus of such models should be understanding how fish might be affected by global warming, Alder added.

"Simple models of climate change suggest fish might move to cooler latitudes, and we don't know how predator-prey relationships will get altered by these changes in the ranges of these fish," Alder explained.