After years of painstaking research encompassing drones, satellite-based survey methods and in-water surveillance from hundreds of sites across the nation, the Perry Institute for Marine Science (PIMS) has released a first of its kind report on the health of The Bahamas’ valuable but often underappreciated mangrove ecosystems.
“In the Mangrove Report Card for The Bahamas, we chose to utilize a ratings scale. For example, Low, Medium, High, or Very High when it comes to mangrove damage. A ratings scale provides a more accurate interpretation of the state of mangroves, which varies from island to island based on natural variability and human impacts,” said Dr Craig Dahlgren, PIMS’ executive director.
An expert in tropical marine ecosystems, he led the local and international team of scientists who undertook the massive mangrove assessment.
“Our experience tells us the public prefers a letter grade. So, generally speaking, a grade of ‘B’ would be accurate for many Family Islands. Abaco and Grand Bahama, because of Dorian, would probably be in the vicinity of a ‘C-’ or ‘D+’ but improving. Nassau would probably receive a D based on loss of coastal mangroves over the past 50+ years.”
The Bahamas’ mangroves are estimated to be worth around $3.2 million per square mile per year, given their ability to protect shorelines, shelter important fishery species and sequester enormous amounts of carbon. In carbon sequestration, mangroves remove carbon dioxide from the air by burying it in sediments via their roots, thereby reducing the impact of global warming. It’s that natural activity The Bahamas seeks to monetize in its carbon credit talks with world leaders.
PIMS drew on its 20-year track record of mangrove restoration and conservation efforts in The Bahamas to assess local mangrove forests. Scientists utilized five key health indicators: area, damage, fragmentation, fish diversity and fish nursery.
The first three indicators were derived from satellite imagery across The Bahamas while data for the last two categories came from in-water assessments of over 265 mangrove systems. The Bahamas is second in the region only to Cuba when it comes to vast mangrove ecosystems.
This nation boasts 612,000 acres of mangroves, most of which are on Andros. Over 57 per cent, however, are dwarf mangroves that provide less value for biodiversity, fisheries, carbon sequestration and shoreline protection than taller mangroves. Their growth, less than three feet in height, is stunted due to the punishing salty environments of their home and a rocky ground with little soil for roots to penetrate.
“These mangrove forests are healthy, but they are just naturally of lower quality for biodiversity and ecosystem services than other places,” explained Dr Dahlgren.
No surprise, the report noted extensive mangrove loss due to coastal development on highly developed islands like New Providence and increasingly severe hurricanes, like Dorian, killing mangroves on Grand Bahama and Abaco.
For the most part, however, mangroves have been relatively stable, and loss has been minimal over the past decade, except in areas where major hurricanes have made landfall such as Dorian’s 2019 passage across Abaco and Grand Bahama, Hurricane Irma’s 2017 trek through the southern Bahamas, particularly Acklins and Crooked Island, and Hurricane Matthew’s 2016 impact on mangroves on Andros.
“In some cases, natural recovery has occurred and continues, but in others restoration is needed to jump start the recovery process,” said Dr Dahlgren.
Scientists observed mangrove fragmentation – split habitats where roads or other obstructions blocked natural water flow – adversely impacted biodiversity and made flooding worse in some areas, placing lives and property at greater risk during storms.
An unsung national treasure, local mangroves had a fish diversity score of Medium, but individual sites varied with 42 per cent of sites having a score of High or Very High. Sixty-seven percent of mangrove sites within the Berry Islands received a score of Very High with an overall island score of Very High. Fish communities in Grand Bahama, Eleuthera and the Exumas were also diverse and were ranked as High overall.
Queen conch, which generates more than $7 million annually for the Bahamian economy were most abundant in the Exumas, while the spiny lobster, a cornerstone of the Bahamian economy employing about 9,000 fishers, were most plentiful in the mangroves of Grand Bahama.
Fish diversity was higher in mangroves that were in close proximity to coral reefs. Approximately, 42 per cent of sites had a species richness score of High or Very High.
As it relates to mangroves’ function as a nursery for young fishery resources, there was more good news in that category. The overall fish nursery index rating for Bahamian mangroves was High, with 42 per cent of sites rated as Medium and 46 per cent as High or Very High.
Mangroves are nursery grounds for key reef species like parrotfish, barracudas, groupers, snappers and grunts. Eleuthera (Savannah Sound), Grand Bahama and the Westside of Andros had mangrove sites with some of the highest nursery index scores making them important nursery habitats.
Conversely, mangroves around New Providence had the lowest densities of grunts, snappers and parrotfish, which is likely due to poor water quality, habitat degraded by coastal development and limited connectivity between mangroves and reefs.
Although recent mangrove loss due to Hurricane Dorian barely affected fish abundance, populations are predicted to decline within three to five years in areas where mangroves are unable to recover.
The report explained: “This is likely due to the fact that the mangrove root systems that fish use for shelter remained intact after the storm in most places even though the mangrove trees had died. These dead roots have become brittle and are starting to break up, however. As this happens, the value of these roots as fish habitat will decrease.”
The report card also delivered an in-depth analysis on the impact of levelling, dredging and filling once productive mangrove creeks to create residential communities like Seabreeze, Coral Harbour and Old Fort Bay.
In total, approximately 37% of New Providence’s coastal mangrove creek area has been lost since the 1950s, according to the Perry Institute of Marine Science.
Presently, Bonefish Pond is the largest remaining coastal creek in New Providence. While Adelaide Creek remains mostly intact its challenged by lack of protection and threatened by encroachment.
In areas where mangroves have been lost or the function of a mangrove systems compromised, their value in supporting biodiversity and providing ecosystem services diminishes as well.
According to Dr Dahlgren, all restoration initiatives should be guided by “the best available science” thus improving the likelihood of success.
“For instance, where old logging roads had causeways that prevented water from flowing into mangrove creek systems, installing a culvert, or even better – a bridge, to allow water flow or opening up sections of the causeway could restore functions of the mangrove system.”
He added: “In places where mangroves were killed by Hurricane Dorian, however, replanting mangroves is needed to help the recovery process take hold. In places like Bonefish Pond National Park where a dredged channel altered water flow, a combination of restoring hydrology and replanting was important.”
Although the marine ecologist admits it may be too late to replace the many mangrove systems that were lost, incorporating mangroves into the country’s climate change plan going forward would provide important ecosystem benefits to The Bahamas.
The Perry Institute engaged a wide variety of partners to create this landmark report card, including The Nature Conservancy, Bahamas National Trust, Waterkeepers Bahamas and Bonefish & Tarpon Trust.
PIMS said its next step is to work together with these partners and local communities to scale mangrove conservation nationwide and compile a high priority list of fragmented mangrove systems; the goal is to reconnect them.
Writer: Tosheena Robinson BSc, MSc
More photo highlights below:
