|Manatees photographed during annual population survey. Credit: FWC|
For the second year in a row, the number of manatees in Florida has reached another all-time high.
Biologists with the Florida Fish and Wildlife Conservation Commission (FWC) reported a preliminary count of 6,250 manatees in Florida during the 2016 statewide aerial survey which is conducted annually in late January. Florida's manatee count has more than doubled in the last ten years.
A team of 16 observers from 11 organizations counted 3,292 manatees on Florida’s east coast and 2,958 on the west coast of the state. This year’s count of 6,250 manatees is similar to last year’s count of 6,063. Both years had very good survey conditions and 2015 and 2016 are the only two years on record for which the count has exceeded 6,000 manatees.
“Weather conditions can greatly impact our counts. Fortunately, the conditions during this year’s survey were very good,” said FWC biologist Dr. Holly Edwards.
Researchers have been conducting statewide aerial surveys since 1991, weather permitting, to meet the state’s requirement for an annual count of manatees in Florida waters. Statewide aerial surveys were not conducted during the winters of 2012 and 2013 due to warm-weather conditions.
“The survey is flown during the cold weather, providing broad-scale information about manatee distribution among warm-water sites that the species requires to survive,” said FWC Research Institute Director Gil McRae.
As a result of the manatee comeback, the U.S. Fish and Wildlife Service says that it is unlikely the manatee population will fall below 4,000 for the next 100 years.
Manatee population boom may threaten Indian River Lagoon seagrass
In addition to manatees, seagrass supports habitats for fish, sea turtles, dolphins, the American Bald Eagle, migratory birds, pelicans and other endangered species that make the Indian River Lagoon their home.
An 800 to 1,200 pound adult sea cow can eat up 10% to 15% of its body weight daily in aquatic vegetation which mostly consists of seagrass. According to a U.S. Fish and Wildlife Manatee Recovery Plan, manatees sometime graze on seagrass which leaves the possibility for regrowth - but manatees also "root" seagrass - meaning the entire plant is pulled and the underwater sediment is disturbed. Based on those consumption rates, an average manatee can consume and/or destroy around 3 acres of seagrass a year, depending on the density of the seagrass per acre.
Save Crystal River, an organization that is advocating for the downgrade of the manatee's status from endangered to threatened, argued to the U.S. Fish and Wildlife Service on Saturday that the increasing manatee population is consuming too much sea grass in King's Bay on Florida's west coast.
In Brevard County, Citizens for Florida's Waterways has also contented for years that the increased manatee population is putting too much pressure on the sea grass in the Indian River Lagoon.
Save the Manatee Club argues that downlisting the manatee is premature and points out that, from 2010-2015, 3,217 manatees died in Florida waters which is over half of the highest manatee count.
Endangered species rebound has caused a seagrass collapse before
Many scientists believe that the rebound in endangered sea turtle populations caused localized declines and/or collapses of large seagrass beds in the 1990's and 2000's. In a scientific publication entitled Effects of excluding sea turtle herbivores from a seagrass bed: Overgrazing may have led to loss of seagrass meadows in Bermuda, research suggests that:
It is likely that the removal of the photosynthetic potential of leaves by grazing sea turtles decreased the production and storage of photosynthate in the seagrasses, slowing their growth and reducing the ability of the seagrasses to recover from unfavorable environmental conditions. This makes the effects on seagrasses of the grazing by sea turtles similar to the effects of severe light reduction.
More manatees increase Indian River Lagoon's nutrient load
The manatees' population comeback resulted in an extraordinary event in 2009. Residents in Vero Beach, Florida witnessed the full extent of the sea cows' voracious appetites' end product when a mile-long stretch of manatee fecal matter closed area beaches.
“I’ve never seen anything like it, and I’ve lived along beaches all my life,” beach-goer Bill Becker told TCPalm. “It was disgusting, but mystifying. It looked like Great Dane poop all along the beach.”
In 2014, the Florida Department of Environmental Protection addressed the effect of the increased manatee population on the nutrient load in the Indian River Lagoon which many Brevard County waterfront property owners, boaters, and anglers blame for the lagoon's plight.
"At the time the seagrass TMDLs were developed [in 2009], manatees were not considered as major nutrient contributors to the Indian River Lagoon because not all the data needed to quantify the manatee nutrient contribution were available. It is worth noting that manatees have been part of the Indian River Lagoon ecosystem for a long time," FDEP stated.
"Based on the Department’s Nutrient and Dissolved Oxygen TMDLs for the Indian River Lagoon and Banana River Lagoon report (FDEP, 2009), the long-term annual average TN [Total Nitrogen] and TP [Total Phosphorous] loads entering the Indian River Lagoon system are about 1511 tons and 216 tons, respectively. The 25 to 109 tons of TN and 2 to 7 tons of TP contributed by manatees only account for about 1.7% to 6.7% of TN loads and 0.7% to 3.0% of TP loads entering the Indian River Lagoon system."
"We have at least got [FDEP's] attention to the subject," Citizens for Florida's Waterways President Bob Atkins said of FDEP's analysis of the manatee's nutrient impact on the Indian River Lagoon. "My conclusion is that seagrass loss is worse [from manatee consumption] than I have calculated and free nutrients are not as bad."