The world's oceans not only have fewer fish these days, but carbon dioxide pollution threatens the survival of shellfish, coral and other hard-bodied sea animals, researchers said in three studies released today.
"The chemistry of seawater is changing in dramatic ways and it's having a significant impact on organisms that live in the water," said Richard Feely, a scientist with the National Oceanic and Atmospheric Administration in Seattle who studied carbon dioxide's effect on marine life.
Meanwhile, another group of international scientists called for overhauling management of the world's fisheries, citing declining fish populations.
Taken together, the reports in today's issue of the journal Science paint a bleak picture of the oceans' ability to sustain current levels of aquatic life.
Numerous studies have documented how rising carbon dioxide levels - largely from the burning of fossil fuels - are affecting climate, human health and vegetation. But scientists are just beginning to understand their effect on the seas.
"Up until recently, the ocean's ability to take up so much carbon dioxide has been seen as a good thing, but it's becoming increasing apparent it can also have adverse effects," said Ken Caldeira, a researcher at California's Lawrence Livermore National Laboratory.
In its research, NOAA analyzed 72,000 ocean water samples collected by scientists around the world between 1989 and 1998.
One study found that since 1800, the oceans have absorbed 118 billion metric tons of carbon, making the seas a "sink" for half the fossil fuel emissions since the dawn of the industrial revolution. And 1.9 billion tons of carbon are being added each year, scientists said.
Most of the carbon remains in the upper 10 percent of the oceans, the researchers said, making surface waters more acidic. Given the size of the oceans, the effects are hard to measure, but lab tests show that the changing conditions could threaten certain aquatic organisms sensitive to ocean chemistry.
"It may affect fish in subtle ways, fish that depend on coral reefs that are affected by this process," said Christopher L. Sabine, a co-author.
Organisms such as corals and mollusks pull carbonate ions from seawater to make calcium carbonate, which forms the basis of their shells. But higher carbon dioxide levels inhibit the production of carbonate ions, which trigger the shell-forming process.
In a second NOAA study, lab tests showed that mollusks, corals and tiny shellfish known as foraminifera had difficulty forming their shells when exposed to the higher levels of carbon dioxide.
When researchers took a pteropod mollusk from the subartic Northern Pacific ocean and placed it in seawater with carbon dioxide levels similar to what they found elsewhere, part of its shell dissolved. But the researchers cautioned that their findings don't establish a direct threat.
The highest levels of carbon dioxide are in the North Atlantic, but as the oceans circulate and water moves around the globe, the high concentrations are expected to move to equatorial areas. Scientists say there's no way to know when the changes might affect other aquatic life.
"We're performing this huge experiment on the global oceans and have very little understanding of the effects on the ecosystems that are out there," Caldeira said.
In a separate report, researchers from the U.S., England and Australia called for a reversal in the way the world's fisheries are managed. Currently, governments regulate the catch of specific species, such as bass or swordfish, depending on their populations. But regulators don't consider the overall habitat or other factors, such as predator and prey populations.
The 17 scientists who produced today's report said overfishing is so widespread that the only way to avert a collapse of fish populations is to regulate entire "ecosystems."
"For striped bass, we don't consider what striped bass eat, or even things like what's out there that eats striped bass," said Edward Houde, a co-author and fish ecologist at the University of Maryland Center for Environmental Science.
Commercial fishing operations use trawlers, gill nets and 50-mile lines of hooks to catch swordfish and tuna. But the huge ships also snag other species - known as "bycatch." As a result, the report says, 27 million tons of fish and other animals are discarded worldwide every year - including sea turtles and prized sport fish such as the white marlin.
The scientists call for setting up "ocean zones" where certain types of activities, such as commercial or sport fishing, would be prohibited. "It's kind of like a land use planning for the oceans," said Ellen Pikitch, the lead author and the executive director of the Pew Institute for Ocean Science.
Federal regulators say they're moving in that direction but have to move carefully, given the effects on habitats and livelihoods. "We don't feel this is something we can do overnight," said Rebecca Lent, deputy director of the National Marine Fisheries Services.
Commercial fishing is a $28 billion industry in the U.S., while saltwater sport fishing nets another $20 billion.
Any changes in fishery regulations are bound to be controversial. For example, two environmental groups are already suing the fisheries service to force it to list the white marlin as an endangered species. But recreational fishing interests say a ban on white marlin would cripple a charter boat industry that returns most of its catch to the ocean voluntarily.
U.S. fisheries are governed by a 1976 law that prohibits foreign fleets from fishing within 200 miles of the coast. Catch limits are set by eight regional councils, comprising state and federal officials, commercial and recreational fishermen, and scientists. Environmental activists say the system is weighted in favor of fishing interests.
The report's authors do not address the regulatory structure, but say an ecosystem-based approach is a necessity, in large part because of habitat destruction and wasteful practices by commercial fisheries.
Last year, the nonprofit Pew Oceans Commission made similar recommendations. Another study, based on 10 years of research, found that since the 1960s, industrial fishing fleets have taken 90 percent of the giant tuna, swordfish, marlin and other large predator fish from the world's oceans.
Overfishing also has decimated the cod population. A 12-year moratorium on cod fishing in much of New England's waters, has yet to spark a rebound, Pikitch said.
"The thing we're learning is, there's not as much room for mistakes as we thought there was," Pikitch said.