With human population growth on the rise, watercraft use has followed suit and also increased in popularity. As more people take to the seas, their impact on marine wildlife becomes a persistent question, especially manatees. T. manatus (Figure 1) are vulnerable to many threats—ranging from natural ones like algal blooms and extremes in water temperature to human ones like encounters with boats and dams. About half of adult manatee deaths are attributed to human-related causes, primarily watercraft collisions. This is significant because the manatee population growth rate is highly sensitive to changes in adult survival rate. Watercraft collisions account for approximately 25% of all manatee deaths and 35% of documented deaths of known cause, and are the single greatest cause of human-related mortality (Deutsch, Self-Sullivan & Mignucci-Giannoni 2008). In 2005, there were over one million registered vessels in Florida (FHSMV 2005), and many more out-of-state boaters visit Florida annually. The number of registered vessels in Florida has increased by an average of 2.9% per year over the past 25 years, doubling since 1980. Given that about 97% of registrations are for recreational watercraft, it can be expected that there will be a continued increase in recreational vessels. This trend follows the growth in the human population (Deutsch, Self-Sullivan & Mignucci-Giannoni 2008).
Historically, humans had almost poached manatees to extinction in ENP (Stith 2006), and continue to be hunted illegally in many other parts of their range (Figure 2).
Manatees are also killed in canal locks and found entangled in fishing nets. They are also threatened by the loss of (or damage to) sea beds due to agricultural and industrial runoff. These same pollutants have been shown to accumulate in the tissues of manatees and some could be toxic to the animals (FPL 1989).
Furthermore, manatees that survive impacts with boats suffer debilitating injuries. Such sub-lethal wounds include heavy superficial scarring and skeletal fractures. The objective of this study is to determine how seriously these watercraft collisions affect the population of T. manatus in Everglades National Park. Once the locations of manatee sightings are recorded, their geographic distribution can be plotted and analyzed. Mortality trends will then be used to evaluate the effectiveness of current manatee protection measures and determine what corrections need to be made to those measures to ensure that T. manatus populations can continue to grow in the future.
Biology of the West Indian Manatee
Scientific Classification (Deutsch, Self-Sullivan & Mignucci-Giannoni 2008)
- Kingdom: Animalia
- Phylum: Chordata
- Class: Mammalia
- Infraclass: Eutheria
- Order: Sirenia
- Family: Trichechidae
- Genus: Trichechus
- Species: Manatus
- Subspecies: latirostris
The Florida manatee (Trichechus manatus latirostris) is the largest extant member of the order Sirenia. It is a large, fully aquatic mammal that shares a common ancestor with elephants and hyraxes (Mooney & Domning 2002). It lacks hind limbs (like Cetaceans) as part of its adaptation to the aquatic lifestyle. Its thick gray or brown skin is covered with a lightly distributed coat of hair. The fur may protect the skin from excessive algal growth. The average adult Florida manatee measures 2.7–3.5 m (8.9–11 ft) in length and weighs 200–600 kg (440–1,300 lb). The largest individuals can weigh up to 1,500 kg (3,300 lb) and measure up to 4.6 m (15 ft) (Wood 1983). Females generally grow larger than males. The manatee’s flippers have prehensile nails that allow it to grasp food while it eats. They feed on about 60 plant species, with sea grasses being a major food source. They have also been known to hunt some fish and small invertebrates. Because manatees feed on abrasive plants, their molars are continuously worn down and replaced throughout life. Their low-nutrient diet contributes to their low metabolism, and is the reason why manatees need to eat five to fifteen percent of their body weight each day. In spite of this demand, the manatee can survive on 25% less energy than a mammal of similar size (Rathbun 1990). Though they generally move slowly, manatees can swim in rapid, short 15-mph (mile-per-hour) bursts.
T. manatus latirostris is a migratory species that spends most of its time scouring the coasts for food. Though they generally live in shallow coastal areas, manatees can tolerate large changes in salinity. This explains their prevalence in rivers and estuaries. They can live in fresh water, saltwater, and even brackish water. The manatee is limited to the tropics and subtropics due to an extremely low metabolic rate and lack of a thick layer of insulating body fat. Manatees cannot survive for prolonged periods in cold water below about 20 degrees C (68 degrees F), and either die or suffer debilitating illnesses from “cold stress” when exposed to such conditions (Stith 2006). In the winter, Florida manatees can be found exclusively in Florida. During summer, these large mammals have even been found as far north as New York City, New York and as far west as Texas.
While manatees have no natural predators, their numbers are threatened by human activities. Because of their low reproductive rate, it is difficult for the species to rebound from a decline in numbers (University of Michigan 2008). Their breeding habits contribute to their reproductive rate. The manatee’s gestation period lasts from twelve to fourteen months, with the pregnant female giving birth to a single calf. On average, manatees that survive to adulthood will have between five and seven offspring between the ages of 20 and 26. As of 2007, 3,300 manatees currently survive in Florida. Approximately 800 live in the Everglades in the winter (Stith 2006). Nearly 100 inhabit the park year-round.
Everglades National Park is a national park in the United States that protects the southern 25% of the original Everglades. Located in Monroe County, Florida, it is the largest U.S. wilderness area east of the Mississippi River. More than 350 species of birds, 300 species of fresh and saltwater fish, 40 species of mammals, and 50 species of reptiles live within Everglades National Park. Thirty-six species listed as threatened or endangered live in the park, including the Florida panther, the American crocodile, and the West Indian manatee. Approximately 100 manatees inhabit the Everglades year-round, though their numbers swell during the winter.
The Everglades are wetlands created by a slow-moving river originating in Lake Okeechobee, fed by the Kissimmee River, and flowing southwest at about .25 miles (0.40 km) per day into Florida Bay. Covering more than 2,400 square miles of marshes and forests (Figures 3 and 4), it is the third-largest national park in the lower 48 states after Death Valley and Yellowstone. The Everglades has been declared an International Biosphere Reserve, a World Heritage Site, and a Wetland of International Importance, only one of three locations in the world to appear on all three lists (Maltby & Dugan 1994). The entire coastal boundary of the park is considered a marine protected area, covering almost 1,400 square miles of marine habitat.
Everglades National Park attracts an average of one million visitors per year (Everglades National Park 2008). Many visitors ride their boats into the park, raising concern for the safety of sea grass beds and endangered marine animals like manatees.
General Materials and Methods
This study relied heavily on GIS, thoroughly discussed which can be defined as an integrated system of computer hardware and software for entering, storing, retrieving, transforming, measuring, combining, subsetting and displaying spatial data that have been digitized and registered to a common coordinate system (Perez, Telfer & Ross 2005).
ArcMap 10 was the software package used throughout the study, and was responsible for all of the displays, transformations, transpositions, and measurements of spatial data. The software was operated on a 2011 Sony VAIO laptop.
Data was gathered from the National Park Service GIS records and the Florida Fish and Wildlife Conservation Commission’s (FFWCC) website. Maps were generated by transforming EE, KML, and XML files into SHP files and then spatially displaying the data in SHP format. The manatee mortality data was transformed into raster format so that trends could be more readily observed.
Maps and Graphs
Statistical Significance of Spatial Relationship among Manatee Sightings (Map c)
Given the z-score of -0.74, the pattern does not appear to be significantly different than random.
Statistical Significance of Spatial Relationship among Manatee Deaths (Map d)
Given the z-score of 2.52, there is a less than 5% likelihood that this clustered pattern could be the result of random chance.
Statistical Significance of Spatial Relationship between Manatee Sightings and Manatee Deaths (Map g)
Given the z-score of 2.65, there is a less than 1% likelihood that this clustered pattern could be the result of random chance.
Discussion and Conclusions
This study focused on the relationship between synoptic manatee sightings and manatee mortality reports. It should be noted that the synoptic points in Map (c) and Map (g) are all manatee sightings that include live and dead creatures. Of the 955 manatee sightings, 300 (>27%) are carcasses, which have a tendency to drift and decay before they are discovered. One should avoid immediately assuming the manatee carcasses originated where they were discovered/reported. However, it should be noted that there is no statistically significant (p=.45 when p<.05 is considered significant, as in not the product of random chance and chance alone) spatial relationship among the 955 synoptic sightings. Those were all distributed around the synoptic flight path, which covered 600 miles of terrain on the Monroe County portion of Everglades National Park.
On the other hand, the spatial distribution of manatee fatalities is statistically significant (p=.0116). The west coast of Everglades National Park is littered with manatee mortalities, distributed in the shallow waters (free of flats) where boat traffic is most intense. The number of reported manatee mortalities is also increasing annually, associated with a variety of causes as seen below.
266 of the 351 reported deaths had undetermined causes (because of decomposition or some other cause) or were not recovered, only verified. It should be noted that the second leading cause of manatee mortality in this survey (Map d) was anthropogenic, attributed to watercraft collisions. It should also be noted that the reported mortality category maximums occurred over the past five years as the number of total deaths peaked in 2010.
The spatial relationship between the manatee sightings and the manatee mortalities is highly significant (p<.01), strongly suggesting that their distribution relative to each other is correlated and not random.
This relates to the topography of Everglades National Park. The southern portion of the park consists of mostly shallow sea grass flats, which interfere with the maneuverability of watercraft and thus keep skilled watercraft pilots at bay. There are also fewer boat access points in the southern portion of ENP. The west coast of ENP has numerous locations for transporting recreational boats to crucial manatee habitat. The previous correlations should be taken into consideration when preparing boating regulations within Everglades National Park.
Deutsch, C.J., Self-Sullivan, C. & Mignucci-Giannoni, A. 2008. Trichechus manatus. IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2
“Everglades National Park”. National Park Service. http://www.nps.gov/ever/parkmgmt/everglades-national-park-visitation.htm 12/12/11
FHSMV (Florida Highway Safety and Motor Vehicles) http://www.flhsmv.gov/dmv/TaxCollDocs/vesselstats2005.pdf. 2005. 12/15/11.
FPL. “The West Indian Manatee in Florida”. http://www.fpl.com/html/kid_manateebook.html. 1989. 12/13/11
Maltby, E., P.J. Dugan, “Wetland Ecosystem Management, and Restoration: An International Perspective”. Everglades: The Ecosystem and its Restoration, Steven Davis and John Ogden, eds. (1994), St. Lucie Press
Mooney, Sharon, and Domning, Daryl P. “Sirenian Evolution” Encyclopedia of Marine Mammals. Howard University, Washington, DC. Academic Press, 2002; Perrin, Würsin and Thewissen. http://www.edwardtbabinski.us/manatee/sirenians.html. 12/14/11.
Perez, Oscar M., Telfor, T.C. & Ross, L.G. “Geographical information systems-based models for offshore floating marine fish cage aquaculture site selection in Tenerife, Canary Islands” Aquaculture Research, 2005, 36, 946-961
Rathbun, G. 1990. Manatees. Pp. 525-528 in B. Grzimek, ed. Grzimek’s Encyclopedia of Mammals, Vol. 4. NJ: McGraw-Hill Publishing Company
Stith, Bradley M., Slone, Daniel H. & Reid, James P. “Review and Synthesis of Manatee Data in Everglades National Park”. US Geological Survey & Sirenia Project. 2006. http://fl.biology.usgs.gov/Stith_et_al_ENP_Manatee_Administrative_Report.pdf. 12/11/11
“Trichechus manatus”. University of Michigan Museum of Zoology Animal Diversity Web http://126.96.36.199/site/accounts/information/Trichechus_manatus.html. 2008. 12/12/11
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