Overfishing is taking a heavy toll on marine predators such as sharks, tuna and swordfish, says a new study by scientists at the University of British Columbia.

The study, published online Monday in the journal Marine Ecology Progress Series, says predator species in north Pacific and Atlantic waters have dropped by more than 90 per cent since the 1950s.

The study found that predator species are also experiencing a dramatic decline in the south seas as those species are caught and sent to northern markets for consumption.

'What happens next when we have nowhere left to turn?'—Researcher Laura Tremblay-Boyer

"It's important because fish are the last food we harvest from the wild at any type of scale," said lead author Laura Tremblay-Boyer. "When you eat fish, it's usually a predator fish. Whether it's a fish stick or you're at the restaurant, it most likely comes from another country."

Tremblay-Boyer, now a PhD student at UBC, said she conducted the research while finishing her master's degree.

Because it's impossible to count fish in the world's oceans, Tremblay-Boyer and her fellow researchers developed a model that analyzed data such as ocean temperature, the presence of algae, and the distance in the food chain that predator species were to the algae, much like a food pyramid.

The scientists also used a global database of fisheries catches between 1950 and 2006.

Tremblay-Boyer said the results were much like checking a bank account balance for the world's oceans.

"The global overview we have is most definitely not showing a great account balance," she said. "It's going down."

Comparable to clearcuts

She also said fishing is a huge driving force in the deterioration of the marine ecosystem, noting that the removal of fish from the ocean can be compared to clearcuts in the Amazon.

"After running out of predator fish in the north Atlantic and Pacific, rather than implementing strict management and enforcement, the fishing industry pointed its bows south," co-author Daniel Pauly said in a media statement.

"The southern hemisphere predators are now on the same trajectory as the ones in the northern hemisphere. What happens next when we have nowhere left to turn?"

Tremblay-Boyer said it's interesting to see how the decline in predator fish has spread from the north to the south, a fact she said will have some very human impacts.

"We have the choice if we're going to eat steak, or tofu or fish," she said. "Some people can only eat fish because they live by the coast. When we eat fish that comes from the south we affect those people."