Dolphins, sharks and other large marine species around the world are going hungry as they seek out dwindling supplies of the small, overlooked species they feed on, according to a new study that says overfishing is draining their food sources.

In a report released Monday, scientists with the international conservation group Oceana said they found several species were emaciated, reproducing slowly and declining in numbers in part because their food sources are being fished out.

"This is the first time that we're seeing a worldwide trend that more and more large animals are going hungry," Margot Stiles, a marine biologist at Oceana and the author of the report, said from Washington, D.C.

"It's definitely starting to be a pattern."

The researchers looked at the health of prey species stocks, like herring, pollock, mackerel, squid and anchoveta, to determine what effect overfishing is having on the larger predators that feed on them.

Using data from the United Nations Food and Agriculture Organization, they found that the majority are over exploited by the fishery. Only 20 per cent of the prey species being fished can sustain the larger predators.

In Canada, scientists said Atlantic cod in the Gulf of St. Lawrence are becoming skinny because they are having more trouble finding reliable sources of small prey like capelin. In Maryland's Chesapeake Bay, striped bass are turning up emaciated because of shrinking supplies of herring and anchovies.

'What we're asking for is that we fish more responsibly so you leave a few fish for the whales, the seabirds and the tuna.'—Margot Stiles, marine biologist

Whales too are having a difficult time finding prey, which researchers say might be affecting their ability and decision to mate. For many endangered whale species, diminished food sources could mean their populations will have trouble recovering.

Seabirds are being particularly hard hit as they choose not to mate because they can't guarantee food sources, Stiles said, citing the example of puffins in Norway where there was a 64 per cent drop in the number of birds having chicks in one year.

Aquaculture greatest source of overfishing: paper

The problem is that as stocks of larger species are depleted, fishermen work their way down the marine food chain and fish smaller prey. Biologists warn that there might be little left in the world's oceans as fishermen fish out the seas.

"What we're asking for is that we fish more responsibly so you leave a few fish for the whales, the seabirds and the tuna," said Stiles.

Landings today of prey species, like blue whiting, skipjack tuna and Chub mackerel, are four times higher than they were in 1950, with more than 10 million tonnes of tiny anchoveta being harvested every year. Seven of the world's 10 biggest fisheries aren't the large, more glamorous species, but the small anchoveta, pollack and Atlantic herring.

In the Mediterranean, that has caused widespread problems for dolphins that are washing up malnourished or sick because of poor nutrition and are more vulnerable to disease.

For fishermen in the Atlantic, the loss of prey species is raising concern that it will affect the lucrative bluefin tuna that feed on herring.

"They're feeding heavily on herring and a lot of fishermen are worried that the herring fishery is taking some of the bluefin's food," said Stiles, whose paper was released in Rome at a UN meeting on the state of the world's fisheries.

The paper says aquaculture is probably the greatest source of overfishing for prey species, accounting for 81 per cent of the small fish that are caught and ground up into meal or oil to feed raised fish.

"It's the primary demand for these fish," she said. "People eat sardines, but that's not what's causing the overfishing — it's the demand for aquaculture."

'Can't take prey fish for granted'

Climate change is also taking its toll on prey fish, which are more sensitive to warming ocean temperatures than their larger predators. So, if the world's waters continue to warm, scientists worry stocks will have even more difficulty recovering.

The researchers are calling for governments to manage the prey species fisheries more carefully, add protections for breeding hotspots so they might be able spawn and refrain from starting new prey fisheries.

"We can't take prey fish for granted any more," said Stiles. "We've always thought they're abundant, that they'll bounce back, but that's not happening. We've exhausted their ability to bounce back."

She said a change in the way we approach fisheries needs to change.

"We need to stop starting up new fisheries simply because we've exhausted all the big fish."