Six million pounds of ancient oyster shells from a Charlotte County mine are now a thriving three-acre reef in the mouth of the Caloosahatchee River.
It’s the latest of about a dozen oyster reefs restored in the region by the Sanibel-Captiva Conservation Foundation that, combined, total seven acres.
The shells, which are tens of thousands of years old, come from a mine near Babcock Ranch.
Each underground shell layer used to be bay bottom, covered under a layer of sand, then under another new bay, over and over on a geological time scale as Florida’s coastline undulates dozens of miles inland and nearly 100 miles offshore from where it is today.
The larger fossilized shell is much like the oysters created today, but heavier, which helps it stay in place during hurricanes. Female oysters actively spawn in the are, and new rules about when, and how much, freshwater can be released from Lake Okkeechobee have helped level out changes in salinity levels in the bay.
That combination of factors are among the reasons SCCF’s marine scientists said they believe the restored reef has high probability of success.
Female oysters release over 100 million eggs during a single spawning event, which are fertilized and develop into tiny larvae that float through the water.
Once the larvae find a hard surface — ideally oyster shell —they attach and settle. Once attached, they're called "spat" and begin growing into adult oysters.
That's why intact shell piles are so critical for reef restoration: the larvae need something hard to latch onto. Without that shell — whether natural, recycled from restaurants, or fossilized — the spat have nowhere to settle en masse so the reef can't regenerate.
“This is a great strategy for cleaning water, it's a great strategy for habitat and species diversity, increasing them,” Eric Milbrandt, SCCF’s Marine Lab, said. “We've lost so many acres."
The three-acre reef is north of Shell Point, which used to be packed with oyster beds. According to SCCF's research, 90% of the oyster reefs at Shell Point were destroyed by road construction and the combined effect of droughts and floods.
Oysters can tolerate typical changes in the salinity of the water in which they thrive.
But at the mouth of the Caloosahatchee River, during droughts, not enough freshwater flows through the region; conversely, when Lake Okeechobee’s flood gates were opened to lower the water level, fresh water flooded down the river and flushed out the saltiness the incoming tide runs up the river.
Milbrandt said now that causeways and other roads are done being built in the area, and with the better flow-control plans in place for releases from the Big Lake, confidence is high the newest reef will thrive.
The hard clams and oysters, including several extinct varieties, are dug up by a company that uses shell to create driveways and for landscaping. The older bivalves and mollusk shells are too big for those purposes, but great for reefs.
Mounds of shells are piled on a barge, taken to the reef site, and a large water cannon blows them overboard in just the right places.
“So you can't just take a big past barge and drop shell there,” Milbrandt said. “You really have to be careful about where you place it.”
SCCF scientists plan to check on the reef every six months to track oyster density and reef development.
Environmental reporting for WGCU is funded in part by VoLo Foundation, a nonprofit with a mission to accelerate change and global impact by supporting science-based climate solutions, enhancing education, and improving health.
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