The world's waters were once seen as a boundless source of fish for humans to eat, but over-fishing and aquaculture have depleted some species and left others famished and weak, according to two reports released on Monday.

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 - such as salmon and tuna - have already been over-fished, the environmental group Oceana reported, adding that now the smaller fish that these fish eat were 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," she said.

When fish stocks decline, that poses a potential problem for humans, according to the United Nations 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 UN organization said in a report on the state of the world's fisheries and aquaculture.

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

The UN report questioned the notion that aquaculture would automatically grow to meet demand, saying this sent a "surreptitious message" that no public policies were 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 them are underfed, sometimes so much so that they can't reproduce or feed their young, the Oceana report said.

With commercially attractive fish such as 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.

(Shanghai Daily via Agencies March 4, 2009)