The population breakdown in the world's oceans is changing dramatically, with big fish numbers dropping while smaller fish become more plentiful, according to a new study.

Villy Christensen, a researcher at the University of British Columbia Fisheries Centre, says that means predators like tuna, cod and swordfish have seen their numbers drop by two-thirds over the past century, while small prey fish such as herring, caplin and anchovies have doubled.

He compares it to human beings visiting Africa's Serengeti region, seeing gazelles and antelopes everywhere, and wondering where the lions are.

"With the big predators gone, the population of prey species would soar. The underwater is an ecosystem just like the Serengeti, where you have predators and prey and a big complex food web."

Much of the decline in last 40 years

Christensen, an ecosystem modeller, and several colleagues presented their findings Friday at a meeting of the American Association for the Advancement of Science in Washington, D.C.

"The UBC team also found that of the decline in predatory fish population, 54 per cent took place in the last 40 years alone," says a news release from the university.

The culprit in global oceans is worldwide overfishing of tuna, groupers and other big, tasty fish that are slow to reproduce. Christensen says this has unbalanced the underwater system and allowed smaller species to thrive.

Fish tastes must change

The solution could include encouraging humans to try more eco-friendly fish such as sablefish, albacore tuna, mackerel or sardines, says marine biologist Mike McDermid.

He's the program manager for the Ocean Wise conservation project at the Vancouver Aquarium, and works with restaurants and the seafood industry to get consumers to stop chowing down on overfished species such as orange roughy and bluefin tuna.

"We need to get away from our traditional tastes a bit — always eating the same thing, always focusing on those big, predatory species — to open up to some new cuisine," McDermid said.

In food lingo, that means eating lower down on the food chain in an attempt to right the underwater imbalance of predator and prey.

With files from Greg Rasmussen