Just a few years after scientists warned of impending ocean apocalypse, a handful of simple management tools have pulled some of Earth’s fisheries back from the edge of collapse, according to a review of global fish populations and catch data. But though the big picture is brighter than before, many of the details remain dark. Some scientists say certain populations may hit “tipping points” beyond which recovery is practically impossible. “In most cases, when you reduce fishing pressure enough, the stock rebounds. But there’s a breaking point beyond which the system has changed so much that it may not recover,” said Boris Worm, a marine biologist at Canada’s Dalhousie University. “The longer you wait to fix a situation, the harder it becomes.”

Three years ago, Worm said Earth’s ocean ecosystems were on the verge of collapse. Nearly one-third of fished species had already been critically depleted. The rest would follow by mid-century. In a paper published Thursday in Science, a Worm-led team of fisheries experts updated those findings, providing the most comprehensive analysis to date of global fisheries. The findings are mixed.

In five of 10 well-studied regions — Iceland, Newfoundland-Labrador, the Northeast U.S., Southeast Australia, and the California Current — fishing pressures have on average become less intense. One-third of the all fish populations have been steered away from imminent doom, and appear to be recovering. Their ecosystems are no longer fast-tracked for collapse. The solutions were relatively simple: abandon destructive fishing techniques like longlining and bottom trawling, reduce catches, put some waters off-limits, and give fishermen an economic reason to not overfish. New solutions didn’t need to be invented.

From those perspectives, the new study is hopeful. But looked at another way, the numbers are grim. There are more collapsed fish populations now than any other time in recorded history. Even within those less-pressured regions, many individual species are threatened. Two-thirds of all stocks need to be rebuilt, and half of those are still being overfished. Some scientists warn that the rebuilding needs to happen now. Pushed too far, some species simply won’t be able to recover, despite our best efforts. They hit a population-level tipping point. It’s hard to know when it will happen, or what the consequences will be.

“Some populations that we’ve tried to rebuild haven’t recovered at the rate we expected. It’s not necessarily the case that we can rebuild all these populations,” said Ellen Pikitch, a marine conservation biologist at Stony Brook University who was not involved in the study. “We don’t know where that threshold lies for most species on the planet. But we do know that if you push a population too hard, you can’t expect it to come back.”

The best example of this comes from the Northeast Atlantic cod fishery. By the mid-20th century, populations of the fish — an economic mainstay, supporting more than 50,000 fishermen — had fallen to dangerously low levels. Fishing was restricted, and the cod quickly recovered. But in the early 1990s, the population collapsed. And today, in the near-absence of fishing pressure, the cod have failed to return. Some researchers think they’ll go extinct within 20 years, no matter what’s done to save them. It’s not known why cod recovered before and not now. It’s possible their numbers fell below some critical density needed to find mates at a population-sustaining rate. Other fish may be eating their young. Another species could have taken over their ecological niche.

Even more worrisome is the possibility that tipping one fish species will set off a chain of reactions that causes an entire ecosystem to flip from one state to another. Known as a critical regime shift, this phenomenon has only recently been described by ecologists, who are now trying to understand it. The shifts involve interactions between multiple species, and are almost impossible to predict.

In the coral reef ecosystem of the Caribbean, for example, overfishing of algae-eating reef fish allowed algae-eating sea urchin populations to explode. The reefs themselves remained healthy, but when a disease destroyed the urchins, algae soon dominated. It choked out the coral, and the reef system collapsed. Another overfishing-linked shift may be happening in the Sea of Japan, now inundated by giant, poisonous jellyfish. Overfishing removed their predators. Commercial fishing has suffered, and Japanese fishermen are now trying to market what was once considered an unpalatable goo. “Once I was talking about eating our way down marine food webs with Daniel Pauly” — director of the University of British Columbia’s fisheries center — “and he said, ‘One day we’ll be eating jellyfish.’ It was meant to be a joke,” said Pikitch. “Then we started seeing jellyfish in stores.”

(Por Brandon Keim, Wired, 30/07/2009)