The current red tide plaguing the entire Southwest Florida coast, along with the blue-green algae in the Caloosahatchee River, illustrate why it’s imperative that we reduce nutrient inputs to our estuaries. As these blooms have festered, everyone has gotten understandably frustrated and discouraged. Over the last several months, a lot of finger-pointing has been happening, but more recently I’m encouraged to see groups forming and ideas for reducing and treating land-based nutrients being developed.
One such idea comes from local clam farmer Barry Hurt, and has the support of other local growers and a number of scientists. The idea is to restore algae-consuming shellfish — specifically our native southern hard clam, Mercenaria campechiensis — to filter the water.
The southern hard clam has a rich cultural and economic history. It was an important food source for the Native American Calusa, as evidenced in their many shell mounds. Many of today’s old-timers also report about the importance of easily accessible clams, and other shellfish, in providing for dietary needs during the Great Depression.
In addition to local sustenance harvest, local hard clams were commercially harvested from the 1880s through the 1960s. Clams were so plentiful in the Ten Thousand Islands, they were harvested 24 hours a day, every day. This fishery began in 1908, peaked in 1932, and was depleted by 1950. Another commercial hard clam fishery was started in Charlotte County in 1962 but was short-lived.
A 1973 survey reported a significant decline in hard clam populations throughout Florida. In addition to overharvesting, several hydrologic changes to our waterways and population growth along the coast are reported to have caused the decline in native hard clam populations.
Southern hard clams are protandrous hermaphrodites, meaning they begin life as males but often change to females (like snook). In Florida, spawning occurs in spring and fall when water temperatures reach about 73°F. The release of sperm into the water by male clams stimulates females to expel eggs. A female may spawn several times each year, producing millions of eggs.
Eggs hatch in 12 to 24 hours, and in less than one day larvae transform to free-swimming animals with tiny wing-like lobes that propel them through the water. At 6 to 10 days, each larva settles to the seafloor and begin growing its shell. Tiny filaments hold the juvenile clam in place. As the clam matures, it will develop a muscular foot, thus enabling it to bury down into the sediments, with only its siphons protruding.
Southern hard clams provide many ecological functions. Here are a few from Barry’s plan:
WATER FILTRATION: A single clam filters approximately 5 gallons of seawater per day. High densities of clams could potentially reduce harmful algae through consumption — including Karenia brevis, the algae that produces Florida red tide.
NUTRIENT TRANSFER: Filter-feeding clams remove algae from the water column and transfer nutrients to the benthic environment (the seafloor). These nutrients may promote seagrass growth and support a variety of organisms that live in the sediment (e.g., brittle stars and worms).
NITROGEN REMOVAL: Clams remove nitrogen from the water by filter feeding. Nitrogen is incorporated into their shells and meat. Clams also stimulate bacteria that are capable of changing nitrogen in the water to harmless nitrogen gas, effectively making it unusable for algae.
CARBON STORAGE: Clams extract carbon from seawater to make their shells. An average clam contains 3 grams of carbon in their shell, providing a source of carbon sequestration.
PHOSPHORUS STORAGE: Clams extract phosphorus from the seawater and store up to one percent of their meat weight in phosphorus.
TURBITY REDUCTION: Filter feeders promote clearer water, allowing deeper penetration of sunlight and boosting sea grass growth. See what just a few clams can do by watching a short filtration demonstration at https://goo.gl/MnbZ1u.
Restoring southern hard clams will not be as easy. Hard clams are prey for just about everything in the water during their early life cycle. To ensure their survival, they’ll need some TLC. But clam farmers know how to raise clams, and over the past 20 years have perfected their growing techniques. Today, they see upwards of 70 percent survival at their lease sites.
Barry’s proposal would use the growing techniques clam farmers know best to restore native clams in suitable areas of Charlotte Harbor. Farmers would be paid upon completion of the various stages of the planting and maturity process, thus providing them economic relief during times when they are unable to harvest their lease sites due to red tide or other water quality closures. Clams from restored sites would not be commercially harvested and recreational harvest will be strongly discouraged.
This proposal is still in its infancy and funding has yet to be secured, but I’m impressed. ClamRestore.com has been recently established to provide those interested with project information and updates. Look for this site to be populated with details soon.