Do the math: Oysters can't be the cure-all for a clean Chesapeake Bay

An oyster-encrusted "castle" placed atop an old breakwater nearly breaks the water's surface in an inlet off Maryland's Choptank River. (Bay Journal photo by Dave Harp)
An oyster-encrusted "castle" placed atop an old breakwater nearly breaks the water's surface in an inlet off Maryland's Choptank River. (Bay Journal photo by Dave Harp)


Many years ago, marine biologist Roger Newell of the University of Maryland Center for Environmental Studies, now a professor emeritus, approximated that a single oyster could filter 50 gallons of water per day (gpd) under ideal conditions and oysters could have once (pre-1870s) filtered the Bay's entire volume every three days.

Then, a number of years later, he re-evaluated and reduced the estimate to 30 gpd, because "ideal conditions" for oysters do not exist everywhere in the Bay and its tidal waters.

Still, many leading Bay organizations have stayed with 50 gpd and show this inaccurate oyster number on their websites. You can see it for yourself on the websites of the Chesapeake Bay Foundation, Coastal Conservation Association, ShoreRivers and Chesapeake Bay Program.

More recently, another UMCES scientist took a closer look and reduced the estimated filtration rate even further. Matthew Gray, an assistant professor and researcher specializing in ecophysiology, was recently quoted in the Bay Journal, saying that when average water quality, water temperature and food availability are taken into account, the filtration rate is likely 12.5 gpd or lower, perhaps as low as 3 gpd.

With such reduced values, let's try some math to re-examine filtering the Bay with oysters. We'll start with 18 trillion gallons as the total volume of water in the Bay, as estimated by the Bay Foundation. Let's be conservative and use Newell's revised filtration estimate of 30 gpd per oyster, which means it would require 600 billion oysters to filter the Bay per day.

Filtering the entire Bay once every three days (Newell's original hypothetical time span for oyster filtration) would require 200 billion oysters. In 1885–86, Maryland's oyster harvest was at a historical peak of 15 million bushels—which, at 300 oysters per bushel, was about 4.5 billion oysters. So, 200 billion is more than 40 times the peak harvest.

The point is, 200 billion is a lot of oysters, even when compared with the record harvest. Accepting the premise that there might actually have been that many oysters in the Bay, you have to wonder: Where would they all fit?

Now ask, "How many acres are needed to hold all of them?" Assuming they could be grown at 10 oysters per square meter (which is conservative, compared with the threshold level of 15 per square meter for a good sanctuary population, per the Chesapeake Bay Program's oyster restoration metrics), this comes to about 40,000 oysters per acre. Dividing 200 billion by 40,000 yields about 5 million acres needed.

If you do the math using the Bay Program's much more ambitious goal of 50 oysters per square meter, or 200,000 per acre, you'd still need 1 million acres of oysters to filter the Bay once every three days. You will find that such a number of acres isn't physically possible in all of the Chesapeake Bay.

The Baywide estimate for actual oyster bottom—where they are growing or could grow—is about 50,000 acres. This is based on estimates by the relevant state agencies, the Maryland Department of Natural Resources and Virginia Marine Resources Commission.

Yet at least 1 million acres to perhaps even 5 million are needed. This amount of habitat is 20–100 times greater than what actually exists. And it cannot be created, maintained or sustained in order for oysters to do the job that would-be Maryland governor Peter Franchot claims (along with the Bay Foundation, Coastal Conservation Association and ShoreRivers) that oysters could do.

Simply put, the acreage needed to clean the Bay with oysters doesn't exist. Even if one considers that the Bay could be cleaned using oysters and other approaches (sewage plant upgrades for example), the point is you still need the acreage to hold the oysters. Those 1 to 5 million acres don't exist. Remember that in this exercise we used the 30 gpd filtration value, not the even less credible 50 gpd mantra.

Now insert the more accurate 3.0 to 12.5 gpd and watch the impossibility explode.

At 12.5 gpd, we'd need 2.4 million acres of oyster habit. At 3 gpd, it's more like 10 million acres, more than twice the acreage of the entire Bay and all of its tidal tributaries.

It is right about here where I should acknowledge that no serious scientist is arguing that all we need to do to save the Bay is put in enough oysters, or that any number of oysters would literally filter the entire Bay every three days. The point is, the 50 gpd figure is mythical. It's not about science anymore; it's about keeping the money pouring into oyster restoration programs—when so much of that money could be better spent on other Bay restoration initiatives.

There is a serious need for Bay organizations and the public to first realistically discuss the acreage available and then the oyster population that realistically might be attained, to estimate how much water oysters might actually be able to filter. But the real answer will not impress the public, advantage the politicos or raise money for nonprofits, which is probably why you haven't seen sensible science and conversation on this question.

None of this is meant to understate the value of oysters; they provide ecological and economic benefits. More oysters are a desirable result, but cleaning the Bay is about far more than oysters. They are only one small component, about which the public has been misinformed.

Certainly, oysters can help the Bay, and they are important for the Bay, the seafood industry and the regional economy. But as for filtering the Bay gin-clear with oysters, that's a pipe dream.

I once heard an experienced oyster biologist suggest that as the public contemplates trying to clean the Bay with oysters, we should work even harder to clean the Bay for oysters. It is incumbent on all of us to find a better way to clean the Bay. And perhaps in doing so remember that watermen can be and are a part of the solution. The Bay is big enough for us all.

Marc Castelli is an artist specializing in watercolor paintings of working watermen, traditional workboats and racing log canoes. He lives on Maryland's Eastern Shore. His views do not necessarily reflect those of the Bay Journal. This commentary first appeared in the July/August 2022 issue of the Bay Journal and was distributed by the Bay Journal News Service.

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