Research to Foster Long-term Manatee Habitat Restoration
Photo Credit: Wanda Bosworth
The Florida manatee (Trichechus manatus latirostris), an iconic symbol of natural Florida, can be found in the waters of the St. Johns River throughout the year. Manatees feed primarily on submerged aquatic vegetation (SAV), and can grow up to 12 feet in length and weigh as much as 3,000 lbs. Their reliance on aquatic vegetation and clean freshwater habitats makes them key indicators of the overall health of the river ecosystem.
Initially listed as endangered in 1967 under a precursor to the Endangered Species Act (ESA), the manatee remains protected under the Marine Mammal Protection Act (1972) and the Florida Manatee Sanctuary Act (1978). On March 30, 2016, the U.S. Fish and Wildlife Service reclassified the species from endangered to threatened, signaling a decreased risk of extinction.
Manatees consume 4-11% of their body weight in vegetation each day, with Vallisneria americana—commonly known as eelgrass—being their preferred food source.
Manatees and the SAV Protective Enclosure Project
SAV is a foundational component of our river’s ecosystem, offering numerous ecological benefits including biofiltration of pollutants, shoreline stabilization, erosion control, oxygenation of the water column, carbon sequestration and storage, and critical habitat and food for both commercial and recreational aquatic species. Unfortunately, SAV in the St. Johns River faces growing threats, leading to a significant decline in these vital underwater grasses – an essential food source for species like the manatee.
In May 2023, St. Johns RIVERKEEPER (SJRK) launched the SAVe our River’s Grasses Expedition to seek answers and solutions to the disappearing SAV of the St. Johns. Visits occur quarterly for a total of five years. The new SAV Protective Enclosure Project deploys temporary, small-scale exclusion fences at 10 sites along the 80-mile expedition route. This initiative was made possible through valued partnerships with Coastal Conservation Association (CCA) Florida and Mighty River Recovery.
The goal of this effort is to simply protect surviving, yet struggling, native SAV and its viable seed bank and rhizome network. The temporary enclosures provide a refuge where SAV can grow above- and below-ground biomass, germinate and disperse, free from grazing pressure. Data collection within and just outside the enclosures will provide insight to chart long-term solutions.
Herbivory Impacts on SAV – What is “Grazing Pressure?”
Consumption or other disturbance of SAV by herbivores—here referred to as grazing pressure—has been studied and recognized as a significant stressor affecting SAV in the Lower St. Johns River. It is one of several factors used to evaluate potential causes of SAV decline. Grazing pressure can be observed through visible damage or disturbance caused by aquatic species that consume or uproot the grasses before they have an opportunity to establish, grow, and reproduce. “Key grazers” contributing to the decline of SAV in the St. Johns River ecosystem include manatees, turtles, blue crabs, and other native species. In addition, invasive tilapia have been observed burrowing into and disturbing SAV beds, further threatening these critical habitats.
Historically, grazing pressure was not a significant stressor because SAV beds were naturally abundant. However, due to other environmental stressors — primarily decreased light availability , salinity fluctuations, and declining water quality — SAV is now unable to grow rapidly enough or accumulate sufficient biomass to restore the once-thriving beds throughout the river. This creates a ripple effect up the food chain depriving aquatic wildlife that depend on SAV of a critical food source.
SAVing Our River’s Grasses, SAVing Our River’s Manatees
If you have any questions about our SAV work, please refer to the Submerged Aquatic Vegetation (SAV) Temporary Protective Enclosures FAQ on our website. You’re also welcome to contact us directly by email at report@sjrk.org or by phone at (904) 256-7591.
Together, we will make a difference.