When Maryland’s oyster season came to a an end a year ago, it produced a dismal total: 50,000 bushels pulled from a Bay that once produced millions of bushels.
It was the worst harvest ever seen in Maryland—until this year’s landings brought in just 23,000 bushels—prompting the state to propose introducing nonnative oysters to bolster the population.
But a recent paper co-authored by a former Department of Natural Resources oyster expert provides an alternative scenario.
The 2003 harvest could have been 1.9 million bushels, according to the paper, published in the Journal of Shellfish Research, if fisheries managers had slashed harvest in the mid-1980s when diseases hit the Maryland portion of the Bay.
By reducing harvest to compensate for disease mortality, a large and sustainable oyster population could have been maintained in medium salinity areas of Maryland, the authors said.
Even controlling harvest in more recent years, after the population declined, would have produced long-term benefits, according to the authors, who reached their conclusions using a population model developed from 16 years of harvest, disease mortality and reproduction data from Maryland.
“We recommend that it is past time to reduce and control fishing mortality specifically to restore the oyster stock,” said the authors, Steve Jordan, a former DNR scientist who now heads the Ecosystem Assessment Branch of the EPA’s National Health and Environmental Effects Research Laboratory’s Gulf Ecology Division in Florida, and Jessica Coakley, a stock assessment scientist with Delaware’s Department of Natural Resources and Environmental Control.
Many scientists have suggested that Bay oysters have been overharvested in the face of disease, but it is the first paper to attempt to model whether reducing fishing would produce benefits.
“That’s really a very important paper,” said Roger Newell, an oyster researcher at the University of Maryland’s Center for Environmental Studies. “Models are never 100 percent accurate, but that really does show the trend of what can happen if you have the right conservation measures.”
Fishery managers have maintained that harvesting oysters was not impacting the stock because the vast majority of diseased oysters were going to die anyway.
Jordan, in fact, said that he saw no harm in harvest levels during most of his time at the DNR, where he headed the Cooperative Oxford Laboratory, until he began developing models to see what management efforts might help meet the Chesapeake 2000 agreement goal of achieving a tenfold oyster increase.
“I had been skeptical that fishing mortality was a big problem until I did this,” he said. “It made a believer out of a skeptic.”
While most oysters do die before reaching the 3-inch market size, many do survive, especially in low– and-medium salinity areas, he said. Not only will some of those continue to live and grow after reaching that size, they will also produce more larvae as bigger oysters have more reproductive potential.
“The idea that if you don’t harvest them, they are all going to die anyway, is not valid, at least on a long-term basis,” Jordan said.
Models, though, are not reality. And others question whether the model described in the paper reflects what is happening in the real Chesapeake.
“You just don’t see that happening in nature,” said Chris Judy, who heads the DNR’s shellfish program. He said native oysters have fared poorly even in many sanctuaries where there is no harvest. “Disease mortality is an overwhelming force that the oyster population hasn’t been able to overcome,” he said.
Mary Christman, a statistician with the University of Maryland who develops population models, called the paper “interesting” but had reservations about whether its assumptions about oyster population growth reflected reality.
The model assumes oysters follow what biologists call logistic population growth, a well-accepted principal that describes how populations expand until they becomes too density dependent, or crowded.
That is a classic underpinning for population analyses, but Christman said it may not be appropriate for a species as heavily impacted by disease, habitat loss and other problems as the Bay’s oyster.
“While it’s a nice simulation study, I think it’s unrealistic for the oyster population in the Bay,” she said. “I think it places a little too much emphasis on fishing pressure as the sole source. There’s just too much else going on in the Bay.”
Jordan agreed that the logistical equation may not accurately reflect the Bay’s population, especially as high levels of reproduction have not always been related to stock size in Maryland.
He and Coakley made adjustments to compensate but, Jordan acknowledged, “the model is only as good as the data and the assumptions that went into it.”
Nonetheless, he said it did a reasonable job of predicting past long-term trends in oyster stocks and landings. The model is not designed to capture year-to-year variations. “I don’t think this model is a total fantasy,” he said.
The model was based on 16 years of records for oyster reproduction, mortality and fishing pressure—a period that also represents a variety of climatic conditions that heavily influence the severity of disease and reproductive success.
Of those variables, humans can only impact fishing pressure, so the model makes estimates of how the oyster population would respond to different rates of harvest.
Based on the model results, the paper said that if oyster harvests had been reduced by 40 percent in 1986 as the disease problem began hitting the Maryland portion of the Bay, the population would have remained stable and grown over time.
Making that reduction in 1986 would have cost the industry $9 million in landings that year, the paper said. But within a few years, catches would have returned to previous levels, and then grown. “The cost would have been repaid many times in sustainable harvests and ecological services,” the paper said.
Projecting into the future, the model assumes the implementation of strategies to set aside sanctuaries and stock them with hatchery-reared oysters. It assumes these actions would reduce mortality by about 10 percent and increase reproduction by about 10 percent.
The model shows that reducing harvest by roughly 40 percent as recently as 2001—the last year of data included in the model—in tandem with the sanctuary and stocking program, would have resulted in roughly a tenfold increase in oyster population by 2020.
The areas that benefit most, according to the model, are medium salinity zones—a large portion in Maryland that includes such areas as the Choptank and Little Choptank rivers, Eastern Bay and the St. Mary’s River. Those areas get moderate to high reproduction in some years, but still experience periods of low salinity that can diminish disease-related mortality for one or more years.
The paper said establishing sanctuaries in those areas and stocking them with hatchery-reared seed oysters—along with reductions in fishing mortality outside the sanctuaries—provides the most promising opportunity to rebuild productive stocks.
The model predicted “little likelihood” of rebuilding stocks in high-salinity areas where reproduction is good but survival is poor. Prospects of significant recovery were also poor in low-salinity areas, where survival is good, but reproduction poor.
For years, the DNR has transported oysters from high-salinity areas to low-salinity sites where they have a better chance of surviving until they are market size. Although that strategy has been criticized by scientists because of its potential to spread disease, Jordan and Coakley said such movement should continue, saying it may provide the best use of oysters from high-salinity areas.
Jordan said it was unlikely that the movement of small oysters was a major factor in spreading disease today. “The truth is, every place in the Bay is infected,” he said. “If you are careful, as DNR had tried to be over a number of years and move mainly 6-month-old oysters, they will have very low, to zero, infection rates” because they won’t have been exposed to the disease for very long.
Oyster populations have dropped sharply since 2001—the last year for which the model included data—and Jordan said it is possible that the oyster population is now so low it will not recover. “That’s been a question for a long time,” he said. “Is there a point of no return?”
Others believe fishery reductions could still help.
“It’s likely that reducing harvest, based on that model, will restore the oyster stocks, albeit slowly,” said Newell, adding that one of the chief constraints for oyster populations is lack of suitable habitat.
“The best oyster habitat for an oyster to set on is clear, clean shell over a living oyster,” he said. “Every oyster that you take out, you reduce habitat for oysters in the future.”
The paper comes amid a debate over whether native oyster populations are in such poor shape that a foreign species, Crassostrea ariakensis, which has shown resistance to diseases in tests, should be introduced into the Bay.
Such a proposal by the states of Maryland and Virginia is the subject of an environmental impact statement, which is also weighing the outlook for the native species. Mike Fritz, living resources coordinator with the EPA’s Bay Program Office, said the new paper should be included in that review.
“I think it’s going to be a helpful document,” Fritz said. “We should give it serious consideration.”
Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, said the study offered some vindication for the group’s call, in 1991, for a moratorium on harvesting oysters and employing watermen in restoration efforts. “Through a much more painful process, we are pretty much to that point now,” he said.
The fishery is virtually closed and watermen employed in helping to bring it back. Just imagine if we started down that road in the 1990s.”
He said the model supports the need to step up to large-scale sanctuary-based restoration efforts to help rebuild populations. “The question is not who was right or who was wrong and what should have been done,” he said. “The question is what can we do now.”