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Hemp Key elevation assessed ahead of restoration work

By SCCF - | Nov 4, 2020

SCCF

Prior to doing any restoration work, the Sanibel-Captiva Conservation Foundation’s Marine Lab recently assessed the elevation of Hemp Key. The elevation survey helped determine whether oyster shell additions or mangrove planting would be needed to increase the mangrove canopy cover.

Hemp Key is an island that is part of the Pine Island Sound National Wildlife Refuge. It has a Calusa shell mound and is used by many bird species for nesting. Aside from the shell mound uplands, the seven-acre island is otherwise all mangroves.

“The island has well-developed oyster reefs on the east side that once formed the margin of an extensive mangrove canopy,” Marine Lab Director Dr. Eric Milbrandt said. “The canopy is mostly gone today but observations and historic aerial imagery show the extensive mangrove canopy.”

Mangroves and oysters do not overlap in their vertical distribution along the slope of a shoreline. Oysters typically are found between -0.25 meters and -0.75 meters, while mangroves are found at 0.0 meters and higher.

“Using a Garmin depth sounder linked to a Trimble GeoXT GPS, the elevations were determined by mounting the sounder to the kayak as shown,” he said. “The vertical accuracy is 8 centimeters for the kayak surveys and about 2 centimeters for the equipment when using with a staff.”

SCCF Digital elevation model of Hemp Key.

The Marine Lab staff surveyed the entire area on Oct. 14 and created the digital elevation model. The team will now use the elevation model to determine how the island will be restored. On Nov. 6, SCCF and Coastal Watch volunteers will begin planting mangroves and adding oyster substrate additions.

The Marine Lab and Coastal Watch will also need volunteers to fill buckets of oyster shell in November and December to deliver up to Hemp Key. Dates will be announced soon.

SCCF