A high-profile article in the journal Science has sparked a debate with Chesapeake region scientists over the role overfishing played in the decline of the Bay and other coastal areas.
The widely publicized article by a team of international researchers contends that the overfishing of certain key species, including oysters in the Chesapeake, preceded other significant forms of human disturbance to coastal ecosystems.
The article in the July 27 issue argues that the historical removal of whales, manatees, dugongs, sea cows, monk seals, crocodiles, codfish, sharks, rays and other large marine animals fundamentally altered various coastal systems, making them more susceptible to ecosystem collapse.
The article, “Historical Overfishing and the Recent Collapse of Coastal Ecosystems,” said that the overharvesting of key species altered food webs, causing impacts that rippled through ecosystems, leading to widespread damage.
For example, the authors contend that vast kelp forests in coastal areas declined because of the overharvesting of cod, whales and other predators that kept populations of sea urchins in check. With predators overfished or hunted nearly to extinction, the sea urchins overgrazed the kelp beds, causing widespread habitat loss.
“A kelp forest is the equivalent of a forest of trees on land,” said lead author Jeremy Jackson, a marine biologist at the Scripps Institution of Oceanography in San Diego. “It disappears because of an imbalance of predators and herbivores, then the whole system crashes.”
In many areas, 500-pound sea turtles, sharks, whales, river otters and other species were far more common in the past than most realize today, and their decline set off ecosystem changes that ultimately led not only to the decline of kelp forests, but also coral reefs and seagrass beds in many areas, the authors said.
One example heavily cited was the Chesapeake, where, they said, the massive overharvesting of oysters in the late 1800s spurred the decline of the Bay’s water quality. Huge oyster population once built giant reefs in the Chesapeake that broke the surface of the water. Their numbers were so great, some believe, they were once capable of filtering all of the Chesapeake’s water in a few days, removing vast amounts of algae. Now, it takes a vastly depleted oyster population about a year to do the same job.
As a result, most of the algae now die and drift to the bottom. Instead of being consumed by oysters and fish, they are consumed by bacteria with rapid metabolisms that quickly deplete the water of oxygen, a process known as eutrophication. The result is huge “dead zones” where most species can’t survive.
In all of the ecosystem declines they cited, the Science article authors said overfishing was the primary culprit — not pollution, climate change or other disturbances, although such factors could complicate the problems.
But many Bay scientists sharply dispute that argument, at least for the Chesapeake.
In a letter quickly published by Science eight prominent researchers from the University of Maryland Center for Environmental Science, the Virginia Institute of Marine Science and the University of Queensland in Australia, took issue with the idea that oyster overfishing was the primary cause of the Chesapeake’s problems.
Although overfishing the filter feeder was likely an important component of the Bay’s decline, they contend that the Bay’s water quality was already worsening in some places in the 1700s as the result of land clearing — long before the massive oyster overharvests of the late 1800s.
And while oyster filtering capacity was reduced by the mid-1900s, “dramatic intensification of hypoxia [low oxygen conditions] and the extensive loss of seagrasses occurred later, during the last half of the 20th century, associated with a more than doubling of the already elevated nitrogen loading.”
The scientists agreed that restoring oysters would help the Bay, something the Bay Program recognized in calling for a tenfold oyster increase by 2010 in its Chesapeake 2000 Agreement. “But restoration of oysters even to pre-colonial abundances is unlikely to eliminate algal blooms and hypoxia and recover seagrasses without also significantly reducing nutrient loading,” the Bay scientists wrote.
They also criticize the Science article for failing to acknowledged the role of oyster diseases in reducing oyster populations — and hindering recovery efforts. The original article suggested that oysters living high on reefs would have been resistant to disease. While that may be true for one of the two main diseases — Dermo — the Bay scientists say that MSX will kill oysters wherever they occur.
“We basically agree that overfishing is a serious problem and does allow ecosystems to go haywire like,” said Donald Boesch, president of the University of Maryland’s Center for Environmental Science, who was the lead author of the letter.
“But,” he added, “if you read the scientific papers carefully and look at the sediment records, it is clear that the first signs of eutrophication began to appear in the late 18th century, well before mechanized harvesting of oysters could have depleted the oyster population.”
Boesch and others said they were concerned that the paper could lead people to believe that the Bay could be restored only through oyster restoration, and not nutrient reductions.
By some estimates, the Bay now gets six to eight times more nitrogen, a nutrient that spurs algae production, than it did before the region was colonized in the 1600s.
“The nutrient problem now is the dominant problem, and it has to be solved before you can restore the Bay,” said Ed Houde, a fisheries scientist with U-M’s Center for Environmental Science, and another signer of the letter. “You couldn’t solve the Bay’s problem now simply by restocking oysters and fishing them properly.”
Oyster overfishing contributed to the decline, Houde said, but the argument that it was the primary cause for the Bay’s decline was “too strongly stated.”
That sentiment was echoed by others, including scientists who were not signatories to the letter.
Steve Jordan, an oyster scientist with the Maryland Department of Natural Resources who is director of the Sarbanes Cooperative Oxford Laboratory, said the Bay maintained — relative to today — a sizable oyster population until the mid-1980s, when MSX became widespread.
But the Bay’s water quality was probably at its worst in the late 1970s and early 1980s, before the oyster population totally bottomed out as the result of diseases, he said.
“It would be hard to say that more oysters wouldn’t be beneficial to the overall eutrophication question,” Jordan said. “But the important question is whether we are going to bank on oysters to solve our nutrient loading problems, or are we going to continue to attack the problem at the source. Some of the scientists I’ve talked to who are very supportive of the role of oysters in helping eutrophication don’t support the idea of trading a land-based approach for oysters.”
Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, agreed that it was unlikely that the demise of the oyster alone caused the Bay’s decline.
But he said it was likely that a healthy oyster population would have helped buffer the Bay from pollution. And the loss of the oyster was further compounded by the loss of other filter feeders which, in effect, served as ecological back-up systems in the Bay.
“When those redundancy factors were removed, particularly when menhaden were overharvested in the past, that was kind of the final straw for the food web that did cause a breakdown,” Goldsborough said.
“The system could absorb a lot more if you had an intact oyster population,” he added. “That’s not to say it wouldn’t be degraded now with all the nutrient loadings that we have dumped into it. It probably would be.”