Climate change is expected to add more stress for fish populations, forcing warm-water species further toward the poles, changing marine and freshwater food webs and habitats, the reports said.

The big fish most likely to appear on rich countries' dinner plates -- like salmon and tuna -- have already been over-fished, the nonprofit environmental group Oceana reported, adding that now the smaller fish that these fish eat are under pressure.

"We've caught all the big fish and now we're going after their food," said Margot Stiles, a lead author of Oceana's report, "Hungry Oceans." "We're stealing the ocean's food supply; these are fish that we basically never used to eat."

When fish stocks decline, that poses a potential problem for humans, according to the U.N. Food and Agriculture Organization.

"The question is whether per capita supplies of fish for human consumption will remain steady or peak in the near future and then start to fall," the U.N. food organization said in a report on the state of the world's fisheries and aquaculture.

In the last three decades, aquaculture has grown rapidly, from about 6 percent of fish available for human consumption in 1970 to about 47 percent in 2006, the U.N. organization said.

The UN report questioned the notion that aquaculture would automatically grow to meet demand, saying this sends a "surreptitious message" that no public policies are needed.

"Aquaculture-enabling policies are essential for the steady and sustainable growth of the sector," the report said.

The drop in the amount of available prey fish -- small, fast-growing species such as herring, sardines, squid and krill -- means predator fish, seabirds and whales that feed on the little fish are underfed, sometimes so much so that they can't reproduce or feed their young, the Oceana report said.

With commercially attractive fish like Pacific salmon and blue fin tuna depleted in the wild, fishing fleets turn to prey fish for revenue where in the past they only used these species for subsistence and bait, Oceana said.

Some of these prey fish are used for human consumption, but increasingly, they are fed to farmed versions of the large predator fish, Stiles said. This in turn means there are less

Climate change could add new problems, both reports said, because prey fish are particularly sensitive to warm temperatures and prey populations have collapsed when heavy fishing proceeded during previous warm periods.

To help reverse the trend, the Oceana report said, existing fisheries need to set conservative catch limits, avoid fishing in depleted species' breeding hotspots and restore the prey fish in the wild to support a comeback of predator fish.

The UN report was released on the first day of a meeting of the global organization's Committee on Fisheries in Rome.