Are artificial reefs good for the environment?

It sounds like a great idea. Take a retired oil rig, Navy ship or fleet of subway cars that would otherwise add to our nation's swelling heap of trash and drop it off the coast of just about anywhere. Plants and invertebrates will colonize the hulking structure. Fish will migrate there and reproduce. And before long, the replenished ecosystem will fuel an economic boon of recreational fishing and diving.

While the concept isn't new, more artificial reefs are being lowered into the ocean now than ever before. Six hundred old subway cars off the coast of Maryland; 125,000 tons of volcanic rock into the ocean near Southern California, and a 524-foot vessel off the Florida Keys are among hundreds of projects slated for this year and next. But as artificial-reef initiatives grow to include more coastal regions and a wider array of aged infrastructure, some scientists worry that the commercial interests of fishing, diving and trash disposal are driving efforts that should be environmental. If that's the case, they warn, thriving reefs may ultimately give way to an underwater junkyard. "Unfortunately it's one of these things where people take a very superficial view--drop something in the water and a bunch of fish come and that's wonderful," says Jim Bohnsack, a fisheries biologist with the National Oceanic and Atmospheric Administration. "The reality is not so simple."

Fish ecologists refer to it as the attraction-production debate, and even proponents of artificial reefing admit it's a tricky issue. Do man-made reefs replenish ecosystems that have been decimated by pollution, overfishing and global warming? Or do they merely lure existing populations away from natural habitats, concentrating them in unnatural ways and making them more vulnerable to overfishing? "In a lot of cases, that question has been answered," says Bohnsack. "But proponents of reefing aren't too happy with the answer, so they ignore it." Research shows  that artificial reefs only enhance fish populations when habitat is the limiting factor. In many cases, it isn't. Overfishing is the bigger culprit, and because they are popular fishing locales, Bohnsack and his colleagues say, artificial reefs only make that problem worse.
Not everyone agrees. By designing reefs to attract transient species, like Black Sea bass, which flit in and out of the reefs, as opposed to species like grouper that prefer to stay in one place, and by implementing fishing restrictions, reef coordinators say they can guard against the overfishing of vulnerable species. "We believe the attraction-production issue is eminently manageable," says Martin Gary, a fisheries ecologist with the Maryland Department of Natural Resources.

Even so, there's the question of safety and durability. The federal government is still cleaning up an ill-fated 1970s attempt at artificial reefing that employed more than 2 million tires fastened together with flimsy metal clips off the coast of southern Florida. The tires came loose from one another during a rash of tropical storms. Since then, they have been rolling around the ocean floor, destroying natural reefs, polluting the water and occasionally washing up on shore. Wooden ships in Florida and junk cars in Alabama have met similar fates.

While Navy vessels and oil rigs have fared better, subway cars have a projected underwater lifespan of just 30 years. Until recently, most states had rejected the idea of dropping them in the water, based on studies that showed they would not be sturdy enough. But after prodding by the New York City Transit Authority, Maryland, Delaware and New Jersey have teamed up to sink hundreds of them in 2008 and 2009. What happens to all those reefs when the cars degrade and collapse? No one can say for sure. "Our top priority so far has been to raise the funds to do this," says Gary. "We are just beginning to put a monitoring plan in place."

Gary acknowledges that there are more durable materials available, but he says subway cars attract tourist dollars. In Maryland alone, recreational fishing and diving contribute more than $1 billion to the annual economy, and a number of fatal diving accidents in ship reefs have not dissuaded tourists eager to dive the subways. On top of that, the need to dispose of these things provides an added incentive: by dumping its retired cars into the ocean, the New York City Transit authority will save an estimated $13 million in disposal costs.