Centuries of excessive hunting on the high seas, besides devastating the populations of whales, sea turtles, sea cows and otters, has set in motion the collapse of kelp forests, coral reefs and other marine habitat essential for sea life, scientists reported today in the journal Science.
A team of 19 scientists from around the globe has concluded that overfishing and overhunting have had far worse effects on coastal marine habitat than pollution or global warming.
For example, California's coastline was thick with fish and kelp forests until urchins, unchecked by otters and predators that had been hunted out, started a destructive pattern of grazing that reduced the coastline to barren rocks and sand.
"A kelp forest is the equivalent of a forest of trees on land. If it disappears because of an imbalance of predators and herbivores, then the whole system crashes," said lead author Jeremy Jackson, a marine biologist at Scripps Institution of Oceanography in La Jolla.
Citing historical accounts of marine life abundance, the report says that recent assessments of losses don't go back far enough to convey a true sense of the enormous decline.
"We all know that the oceans are overfished and there used to be a lot more out there," Jackson said. "But when we started looking into historical records, it exceeded all of our imaginations."
Jane Lubchenco, a marine ecologist at Oregon State University, said she found the report very startling. "It provides the first credible analysis of the magnitude of human impacts on the ocean."
Poring over records of sediment from the ocean floor, archeological digs and historical harvests, the scientists rediscovered oceans so teeming with whales, sea turtles and fish that they seem akin to a world fantasized by Jules Verne.
In the Caribbean, for instance, sea turtles so crowded the bays that Christopher Columbus worried that his ship would run aground on them when he discovered the New World.
Millions of green turtles used to closely crop turtlegrass. Now that the turtles are largely gone, the turtlegrass in Florida Bay is being consumed by a fungus that depletes oxygen needed by fish to survive.
In Chesapeake Bay, oyster beds once were so thick they posed navigational hazards. The mollusks also filtered the bay water so quickly that the Chesapeake was crystal clear rather than its current murky green.
Since the over-harvesting of oysters, the altered water chemistry has made the Chesapeake inhospitable to the once abundant populations of manatees, giant sturgeon, whales and alligators, according to the report.
Along the Pacific Coast and the Aleutian Islands, sea otters once kept sea urchins in check with their insatiable appetite for the spiny creatures.
Beginning with the native Aleuts and finishing with 19th century fur traders, the otters were hunted to the brink of extinction.
That allowed the urchins to munch their way through the kelp forests--a phenomenon that spread later to Southern California after other urchin predators, the lobster and sheepshead fish, had been overfished, said Jim Estes, a marine ecologist with the U.S. Geological Survey.
Such a change has sharply diminished the kelp forests that provide food, shelter and breeding grounds to so many creatures.
"What amazed me is the same thing has happened in the Gulf of Maine," Estes said. The Atlantic cod were important predators of urchins, he said, until they were nearly wiped out by overfishing. In their absence, exploding urchin populations laid waste to sea vegetation.
In case after case, the scientists and historians found that once one strand in the food web was removed by overfishing, the entire ecological system began to unravel.
Removal of key predators or other animals has set off sequences of events that are now culminating in blooms of toxic algae, ocean dead zones, outbreaks of disease and other symptoms of ecological instability.
Steve Gaines, director of the Marine Science Institute at UC Santa Barbara, believes that the scientists have pointed out a fundamental flaw in government attempts to manage fishing industries.
State and federal regulators impose catch limits, or quotas, that often are based on estimates of how many of a certain species would be in the ocean--if not subjected to fishing pressure. But such estimates reflect relatively recent populations that were already substantially reduced, suggesting that quotas merely tinker with tiny, remnant populations.
"I don't think people realize how the ocean has changed historically," Gaines said. "If you contrast what's out there now to what was there 200 years ago, it's just crumbs."
Gaines is part of a group of California biologists designing a network of marine reserves along the state's coastline--areas to be off limits to fishing--to give species a chance to recover.
Jackson said such efforts are laudable. But he believes that they have little chance of restoring what once existed in the ocean, without extensive human effort to re-create through captive breeding programs what has been lost.
On a bright note, the article maintains that there have been very few documented cases of outright extinction of marine species. So most creatures still exist somewhere in the ocean. That comforting information, however, does not give the scientists cause for complacency.
"We need to intervene and need to intervene in a massive scale," Jackson said. "But we have a problem. We do not have viable science on how to put Humpty Dumpty back together again."