January 9th, 2011 by Michael Ricciardi
In concluding the International Year of Biodiversity, the good news is that manatees are still with us, the bad news is that manatee deaths in U.S. waters continue to climb. In 2009, there were 429 reported manatee deaths, which was about double the number from 2008. As of December 2010, however, manatee deaths totaled 699.
Of the several factors that can cause manatee deaths, the most common one is human-caused: boat collisions. According to the Florida State Fish and Wildlife Service (FWS), 97 of 2010’s manatee deaths were due to boats or other water craft. That number seems consistent from year to year.
However, another death-factor has entered the survival game and is out-pacing boat strike deaths; a growing number of manatee deaths are the result of hypothermia — manatees experiencing “cold stress” resulting from entry into/contact with much colder waters. The FWS estimates that 244 of last year’s manatee deaths (nearly one third) were due to exposure to cold water.
Manatees are mammals, and warm-blooded, but they are not as protected against cold water as other large sea mammals, like seals and walruses. Lacking sufficient body fat, these gentle “sea cows” are highly adapted to warm, tropical waters. Temperatures below 68º F (20º C) can be lethal to manatees.
Coastal development along Florida’s southern (Atlantic) coast line is believed to be pushing the manatees further north where they are more likely to come in contact with colder currents, such as come from sea out-flows from rivers. The Banana River, for example, discharges 50º F water into the coastal marine waters and canals along Satellite Beach– that’s about 25º F colder than the average temperature of the ocean water.
Power plants along Florida’s northern coast discharge warm water effluent into the coastal waterways, creating an enticement for the sea cows. Indeed, in early December, some 300 manatees were counted congregating around the Big Bend Power Station’s discharge canals. But these pockets of warm water provide a false sense of security for the mammals. As they venture outside these locally maintained warm spots, they encounter sudden temperature drop-offs — which they are quite unprepared for.
In some areas, conservation efforts now include artificially heating certain lagoons to keep the animals warm over the winter months. One such place is the The Indian River Lagoon. The lagoon is the former home to a power plant whose regular discharges of warmed water most likely attracted the manatees there in the first place, going back 40 years. According to one local news station, Florida Power and Light is spending over 500 dollars an hour heating the lagoon for the manatees.
The majority of 2010 manatee deaths occurred in January during a 12-day cold snap that was Florida’s worst since 1940. This cold snap reached much further south, where it impacted normally safe manatee habitat. Fortunately, most manatees seem to be staying well south this winter, with one large gathering of 800 animals (the largest ever recorded in one fly over) in the waters off Palm Beach County. Waters are warmer there, but for that same reason, pleasure boating is pervasive. Warnings to be on the look-out for manatees have been issued to most marinas in the region.
It seems that the specially adapted, vulnerable manatee is caught between a boat and a cold place.
Manatees (members of the genus Trichechus) are mostly herbivores and consume up to 10% of their weight in plant matter per day. Occasionally, however, they will eat small amounts of fish that have been caught in nets.
Although they are sometimes preyed upon by large predators like crocodiles and sharks, most of the dangers faced by these gentle mammals come from people, in the form of habitat destruction, pollution (oil spills) and boat/ship strikes. A large percentage of these animals possess propeller scars on their backs.
Manatees are also highly susceptible to red-tides, which consists of large-scale blooms of dinoflagellate algae that secret a neurotoxin that can damage their central nervous systems. In 1996, a particularly severe red tide was believed to be responsible for the decimation of 15% of Florida’s west coast manatee population.
Their territory is relatively extensive (see map above) being found on both sides of the Atlantic and always within a range hovering above and below the Equator. They can grow up to 12 feet in length and reach weights of up to 1200 pounds. In animal studies, the manatees showed comparable intelligence to dolphins and pinnipeds (seals, walruses) and showed signs of a complex associative learning and task discrimination capacity, as well as an advanced long-term memory.
The animal used to be hunted for its skin and meat, and, although hunting of manatees has been outlawed for more the 100 years, poaching continues to this day. The IUCN list all three species of manatee as vulnerable to extinction.
For more information on manatees or to join the conservation effort, visit the Save the Manatee Club website.
Some information in this article came from the SciAm article ‘Another Record Year for Manatee Deaths’, by John Platt
* Wildlife Trust is now EcoHealth Alliance
Top Photo of Young Manatee: Mwanner; CC – BY – SA
Other Photos/Images: as credited, or public domain
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