Have you ever ventured to the Florida coasts and warm water estuaries? If the answer to that question is yes, then you may have been lucky enough to see a Florida Manatee (Trichechus manatus latirostris) or the Florida Sea Cow, as it is sometimes called due to the large quantity of seagrass it likes to eat. Manatees are large aquatic mammals that are distantly related to elephants. They’re also huge! The average manatee is about 10 ft long and 1,200 lbs! While their large size is impressive, it also makes them especially vulnerable to injuries caused by collisions with boat propellers2. Currently, the biggest threat to manatee survival in Florida is boat collision. Florida Manatee is currently listed as threatened under the Endangered Species Act (ESA) and is protected under Florida State law2. Florida Manatee populations have risen over the past 25 years thanks to their listing on the ESA and actions taken by the Fish and Wildlife Service to protect them, but there is still doubt of how the Florida Manatee will fare in the future as old threats worsen and new threats arise.
A Florida manatee “strikes a pose” with its aquatic neighbors at Three Sisters Springs in Citrus County, FL: part of the Crystal River National Wildlife Refuge. Photo: Keith Ramos, USFWS.
So what other threats do these big loveable creatures face in Florida? Florida Manatees can’t tolerate temperatures below 68ºF and in Florida winters coastal water temperatures can fall below 61-64ºF2. Basal Metabolic Rate (BMR) is the rate of energy expenditure per unit time by an organism at rest. Manatees experience elevated costs of basal metabolism when they are in temperatures below their thermal tolerance limit of 68ºF due to an increase in energy being used to maintain their internal temperatures. This increase in the amount of energy being used to maintain a stable temperature can result in metabolic energy production being insufficient to meet the energetic demands of the basal metabolism and can result in mortality3. To survive the cooler winter periods, Florida Manatees utilize several different kinds of habitat which are areas with natural warm-water springs and warm-water discharges from power plant outfalls2. They will also use thermal basins, which are deep holes that trap warmer waters within them for a brief period of time. All of the power plant outfalls that the Florida Manatees currently use to keep warm during winter were built in the mid-1900s, which means that most of these plants will soon be reaching the end of their operational lives2. New power plants will be built to replace those that get retired, but regulations under the Clean Water Act will prevent the new power plants from discharging water that is warmer than the receiving waterbodies2. What does this mean for the manatees that depend on these areas for winter survival? It means that manatees may not be able to find enough suitable wintering habitat and that even if they do find alternative sites, those sites may not be able to support the large number of manatees that get displaced2. This could lead to a reduction in the Florida Manatee population through decreased survival and reproduction as a result of inefficient supplies and wintering habitat.
Florida manatees aggregating at Blue Spring, Volusia County, Florida to thermoregulate. (Photograph Courtesy of the Daytona Beach News-Journal/David Tucker.)
The Natural warm-water springs that the Florida Manatees use for winter survival also face an uncertain future2. The withdrawal water from these springs for domestic, industrial, agricultural, and other purposes could potentially decrease the flow rate of these springs. These reduced spring flows could lead to a reduction in the size of thermal plumes (the warmer water that comes from the springs and has not yet equalized temperature), which could hinder the springs ability to support current manatee numbers2. You’re probably thinking, “couldn’t manatees just move further south and inhabit warmer waters there?” That’s a great question! The problem is that Southern Florida lacks a sufficient amount of warm-water springs and the water temperatures there also periodically fall below the manatee’s thermal tolerance limit of 68ºF2. The warm-water springs in central and northern Florida are the most abundant and provide the best winter habitat for Florida manatees. The loss or impairment of these warm-water springs could lead to a decline in the Florida Manatee Population.
It may seem like the odds are stacked against the Florida Manatee population, but there are steps that can be taken to help prevent the decline of this gentle Sea Cow. One step could be to raise money to go toward the “repowering” of the power plants built in the mid-1900s that produce the warm-water discharge that Florida Manatees depend on for survival. The “repowering” of these old power plants is an expensive process that involves replacing the existing generating units for more efficient ones. If enough money was raised to “repower” these older power plants or the government were to fund their “repowering” then the Florida Manatee Population would not lose this important winter habitat. Protection of the warm-water springs that support the largest number of manatees could also be a step toward preventing their decline. To decrease manatee mortality and injury due to boat collisions, actions should be taken to slow boats down in areas of high manatee density. Incorporating a reduced speed limit for vessels will provide manatees with more time to notice the vessels and avoid collision with them1.
References
- Nowacek, Stephanie M, et al. “Florida Manatees, Trichechus Manatus Latirostris, Respond to Approaching Vessels.” Biological Conservation, vol. 119, no. 4, 2004, pp. 517–523., doi:10.1016/j.biocon.2003.11.020.
- Laist, David W., and John E. Reynolds. “Florida Manatees, Warm-Water Refuges, and an Uncertain Future.” Coastal Management, vol. 33, no. 3, 2005, pp. 279–295., doi:10.1080/08920750590952018.
- Stith, Bradley M., et al. “Temperature Inverted Haloclines Provide Winter Warm-Water Refugia for Manatees in Southwest Florida.” Estuaries and Coasts, vol. 34, no. 1, 2010, pp. 106–119., doi:10.1007/s12237-010-9286-1.