EMERALD ISLE — “Rare Manatee Seen in Outer Banks Marina” proclaimed a headline in a June 2018 edition of the Raleigh News & Observer. In recent years, reports of manatee sightings in North Carolina in the state’s media have become particularly notable not because of their rarity, but because of their increasing frequency.
This increase – particularly since 2010 – might be chalked up as yet another sign global warming enabling tropical species to expand their ranges further north. In this case, however, a more important factor may be successful recovery efforts in Florida that have enabled the size of the Florida manatee population to increase.
Now estimated to number some 8,800 animals, up from perhaps 1,000 to 2,000 in the early 1980s, there are simply more Florida manatees available to roam north as Florida’s winter chill gives way to warmer temperatures in spring. Because manatees can’t tolerate water s colder than about 68 degrees for long periods, virtually all manatees in the southeastern United States, including North Carolina, retreat to Florida to overwinter.
Water temperatures throughout most of Florida also regularly dip to the low 60s or colder for weeks or at least days at a time in most winters. In some years these temperatures are reached even in southernmost Florida. Unlike areas farther north, however, the waterways in central and southern Florida have small, localized areas called “warm-water refuges” where water temperatures typically stay at or above 68 degrees during even the coldest winter days. These refuges rarely exceed a few acres in size, and are often no more than a few tens or hundreds of square feet.
Without warm-water refuges, even manatees in Florida would probably be unable to survive. All Florida manatees therefore learn to find and return into them whenever winter water temperatures dip into the mid-60s.
Manatees are so adept at detecting and following the most minute temperature gradients, Chip Deutch, a manatee biologist with the Florida Marine Research Institute, quipped that “manatees act like heat-seeking missiles” when cold weather sets in.
On the coldest days, up to 80% of all Florida manatees pack into about 15 major warm-water refuges to thermoregulate, or regulate their own temperature, and wait out passing cold fronts. Most refuges are natural springs or power plant outfalls that constantly discharge water 68-70 degrees or warmer.
In southernmost Florida, however, “passive thermal basins” also serve as refuges. These basins are generally smaller areas formed by local hydrological conditions that trap pockets of warm water for at least a few days, or areas with a surface lens of less dense freshwater that insulates a deeper layer of warmer, denser salt water.
Manatees also possess a truly remarkable talent for navigation. They act as if they have onboard GPS systems like those we use in cars to map routes and find the nearest gas station or restaurant. Once manatees find a good source of food, fresh water or warm water, their locations seem to be etched in their memory for future use whenever they happen to be in the neighborhood. Photo identification studies for example, reveal that most manatees faithfully return to the same warm-water refuges winter after winter despite widespread dispersion once water temperatures rise in spring.
Read the rest of the story in Coastal Review Online here: