"Obviously we don't want to shut down fisheries," said the study's lead author, Ben Halpern, a researcher at the University of California, Santa Barbara. "But it's important to recognize that if we want to have healthy kelp forests that support these fish, it's important to have a long-term perspective.""We need to not remove the predators."
Two factors are thought to influence the health of coastal marine ecosystems such as the massive kelp forests: polluted runoff laden with phosphate and nitrogen from fertilizers used in urban and agricultural areas, and overfishing.
The nutrient-laden runoff is thought to encourage kelp, much like fertilizing the garden helps plants grow. But Halpern and his colleagues at UC Santa Barbara's National Center for Ecological Analysis and Synthesis found that benefit pales against the threats posed when fishermen strip the top predators from the ecosystem.
"It's the removal of predators and lobsters that really tips the scales," he said. "You get a domino effect down the chain from removing predators, and you get almost no cascading effect up the chain when you add nutrients."
Other scientists concurred with Halpern's findings.
"That doesn't surprise me," said Steve Ralston, a biologist with the National Oceanic and Atmospheric Administration who works on rockfish assessment off the California coast and advises the Pacific Fisheries Management Council, which sets fishing limits.
The findings imply regulators should exert strong limits on fishing, he added. "I would agree with that, and that's what we're doing," he said.
Indeed, the fisheries council three years ago slammed shut much of the fishing in federal waters for rockfish and other long-lived sport fish, saying stocks were dangerously depleted and could take up to 100 years to rebuild.
More recently, the state has proposed banning or restricting fishing for upward of 25 percent of the coast from Point Conception, south of Big Sur, to near Santa Cruz. Those areas would be part of a newly expanded marine sanctuary system aimed at rebuilding ocean stocks.
The study is the first of its kind to contrast inventories of ocean species taken by scuba divers with images of water quality taken by satellite.
Halpern and his colleagues analyzed 20 years of data detailing population levels of 46 species living in and around kelp forests off the Channel Islands, near Santa Barbara. The inventory is part of an annual census conducted by the National Park Service.
The scientists compared the Park Service data with satellite images showing phytoplankton blooms a distinct sign of nutrient-rich runoff.
The results were stark: The removal of key predators such as the spiny lobster, the kelp rockfish and the Kellet's whelk had seven to 10 times more impact on the amount and variety of kelp forest animals than the increased amount of nutrients deposited into coastal water by fertilizer runoff.
Kelp forests without those top predators had a "totally different suite of animals" in the water column, Halpern said.
Lobster and rockfish are two highly sought species. The whelk a large snail has only recently gained the attention of anglers and consumers in Southern California.
"People haven't been paying attention to this whelk because no one's ever fished it," Halpern added.
"But our results suggest we should start paying attention and maybe set up some regulations for these fisheries as they are starting up."