by MARC CASTELLI, for the Bay Journal News Service
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.
More recently, another University of Maryland Center for Environmental Studies 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. Simply put, the acreage needed to clean the Bay with oysters doesn’t exist.
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.
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.
PHOTO: 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)