. The open ocean is the world's largest ecosystem.This deep blue water far from land is where the most commercially popularfish are caught. Two years ago, scientists made the startling announcement that 90% of all large fish had disappeared from the world's oceans in just 50 years. Now the same researchers report that half the big fish species have disappeared over the same period of time.

The study analyzed data from Japanese longlines, the most widespread fishing gear in the open ocean. Researchers also examined scientific observations from United States and Australian government agencies. The study is the first to examine the impact of climate change and industrial fishing on a global scale.

Dalhousie University biologist Ransom Myers mapped the hotspots where large marine predators like tuna, marlin and swordfish congregate and writes about them in the journal Science. "Off of Florida is one. South of Indonesia is another," he says. "Actually the whole Indian Ocean is incredibly diverse. And, south of Hawaii is another one of these diversity hotspots."

Mr. Myers says the hotspots are shrinking. For example, the ocean off Northwestern Australia, once home to the most diverse collection of tuna and billfish species, is now indistinguishable from the rest of the ocean. "For the same number of hooks you set out, you now get one-half as many species as you did fifty years ago," the biologist notes.

Co-author Ransom Myers says the data indicate a dramatic shift in fish population and the decline of large predator species. He says those changes have a ripple effect. "You knock out the big predators and generally, unusual and unforeseen and bad things happen to humans," he says. "So, you knock out hammerhead sharks and you will get an increase in stingrays that eat lobster and you may care about eating spiny lobster. On coral reefs if you take out the coral reef fish and the reefs will overgrow with algae and die and so protecting these large fish is essential for protecting the ecosystem."

Ransom Myers says environmental changes affect fish populations year to year, but over-fishing is the major force behind the decline in bio-diversity. He says the consequences -- while not completely understood -- could be huge.

"Species diversity is really, really important because it is that diversity among species and within species that we need to allow fishing to take place, to allow the variability to allow the ecosystems to respond to change," the biologist says. "By over-fishing we are removing that variability. Thus the ecosystems will not be able to respond as they need to as climate changes and fishing occur."

But Ransom Myers says the trend can be reversed, and the study can help identify areas for greater protection, "areas of the open ocean where there is no over fishing and that are protected from fishing to allow these species to increase and maintain themselves." If they are protected through international agreement, Mr. Myers says, "They can be optimally harvested and give us maximum protein for the food and maximum diversity and enjoyment for sports fishing, and enjoyment just to know that the world is a better place to have these magnificent fish in it."

The study in the journal Science is part of the Census of Marine Life, a $1 billion 10-year international effort to identify the species in the world's oceans.