It has been speculated that during the winter manatees may be limited to foraging in close proximity to a thermal refuge due to their reliance on warm water. If this is typical manatee behavior it would be reasonable to assume that as the winter progresses food resources near the refuge would become depleted, resulting in manatees moving progressively farther from the refuge to foraging sites. In response to this speculation a literature review was conducted of published research related to manatee foraging behavior to determine if any stereotypic winter foraging habits have been documented.

After reviewing a number of foraging studies, it appears that when and where manatees feed, while reliant on warm water, depends on several factors. There is ample evidence that the proximity of forage to warm water sites is a factor primarily when water temperatures are coldest, or in areas which experience extreme cold water temperatures relative to manatee survivability. In these cases, manatees either do not leave the warm water sites to feed (Rose and McCutcheon, 1980 and Shane, 1984), or as Zoodsma (1991) noted in her thesis on distribution and behavior of manatees in southeastern Georgia "the proximity of feeding areas to warm-water refuges is critical for the survival of manatees in Georgia or northern Florida during the cold season." Conversely, in south Florida where water temperatures are not as extreme, manatees travel significant distances from the warm water refuge to forage. Rose and McCutcheon (1980) noted this in their power plant research, "It appears that manatees will forego feeding in time of severe cold to remain in association with the warmer waters of these effluents, but will also move relatively great distances in search of food in response to warmer air temperatures." Shane (1984) also noted that manatees left the warm water effluent only during warm spells to feed on the eastern shore of the Indian River and otherwise were relegated to the warm water refuge. Similar findings have been made at natural warm water refuge sites. Wayne Hartley (personal communication) has observed manatees "testing" the waters of the St. Johns river only to circle back and remain in the warm Blue Spring run during periods of extreme cold. As ambient river waters warm, manatees have been observed moving rapidly to foraging areas such as Lake Beresford, which is approximately two miles from the spring, where they consume aquatic vegetation and rapidly return to the warm water area. Such foraging forays occur during the warmest part of the day, from the early afternoon to early evening.

Although proximity and water temperature are important there appear to be mitigating circumstances. Packard (1981) noted that manatees traveled to major feeding areas around the FPL Riviera Beach power plant. These areas, in order of importance were: Jupiter Sound, Hobe Sound, Lake Worth, the Loxahatchee estuary and Peck Lake. Tides were noted as an influence to manatee foraging behavior at Jupiter Sound and Lake Worth. Packard (1981) reported, "During cold-induced aggregation at the Riviera Beach power plant, manatees fed in seagrass beds in Lake Worth (5 km north) and Jupiter (25 km north). These areas were close to inlets, where tidal water buffered the drop in water temperature that occurred in the intracoastal waterway. When the ICW warmed to above 20? C, manatees fed in Hobe Sound and the Loxahatchee estuary. Additionally, Lefebvre and Powell (1990) found that Hobe Sound appeared to be preferred over Jupiter Sound by manatees during a mild winter even though it was further from the power plant than Jupiter Sound. Zoodsma (1991) also noted a tidal influence on foraging behavior, but for a different reason. Manatees in southeast Georgia and northern Florida feed primarily on Spartina alterniflora during high tides. Foraging during high tides in this region is a prerequisite for access to this food source.

Notable secondary factors reported in the literature as influences on winter foraging behavior are: air temperature, species of vegetation, biomass, nutritional value of forage, the amount human activity in the area, availability of freshwater and resting areas. Rathbun described the following behavior in northwest Florida, "Because of the large biomass of aquatic plants available as food to manatees in Kings Bay (Etheridge et al., 1985) and Blue Water (Rathbun and Reid, personal observations), relatively little attention has been given to the dietary requirements of aggregated manatees. As a result of our radio-tracking studies, we learned that manatees in both the Homosassa and Crystal rivers frequently left the warm headwaters during the coldest months to feed on Ruppia maritima and Potamogeton pectinatus downriver, despite the abundance of other plants near or in the warm water (for vegetation maps of Kings Bay, see Hartman, 1979, and Kochman et al., 1983). These downriver feeding areas should receive protection so that manatees feeding in the shallow mud flats will not be struck by boats. These feeding areas should also be protected from development. Research should be initiated to determine why manatees are attracted to food plants that are located away from their warm-water refuges." Levebvre and Powell (1990) also noted that manatees may have preferred feeding in Hobe Sound due to "less boat traffic, accessible grass beds much further away from the main channel (thus removed from speeding boats) and more quiet areas for resting and other activities."

Although there is not an overwhelming amount of data on manatee foraging behavior during the winter, the available information does provide some insight. There does not appear to be a stereotypic foraging pattern for manatees that are dependent on warm water refugia. However, water temperature does appear to be the main influence affecting manatee feeding activity during the cold season. During severe cold fronts manatees may not leave the warm water refuge to forage. While during warm periods they may travel significant distances to preferred feeding areas and even between warm water refuge sites. Additionally several secondary factors seem to affect the preference manatees exhibit for specific foraging sites once the deterrent of severe cold water is removed.

Contact Ron Mezich if you have questions about the above article
Florida Fish and Wildlife Conservation Commission
Ron.Mezich@MyFWC.com
(January 20, 1999)

Bibliography

Rose, P.M., S.P. McCutcheon 1980. Manatees (Trichechus manatus): Abundance and Distribution in and Around Several Florida Power Plant Effluents. Final Report, Prepared for The Florida Power & Light Company.

Kochman, H.I., Rathbun, G.B., & J.A. Powell. (1983). Use of Kings Bay, Crystal River, Florida by the West Indian Manatee (Trichechus manatus). Pp.69-124 in Packard, J.M. (ed.). Proposed Research/Management Plan for Crystal River Manatees. Vol. III. Compendium. Technical Report No. 7. Florida cooperative Fish and Wildlife Research Unit. University of Florida, Gainesville, FL. 346 pp.

Rathbun, G.B., Reid, J.P., and Carowan, G., 1990. Distribution and Movement Patterns of Manatees (Trichechus manatus) in Northwestern Peninsular Florida. State of Florida, Department of Natural Resources, Florida Marine Research Institute, Publication Number 48, December 1990.

Shane, S.H. 1984. Manatee Use Of Power Plant Effluents In Brevard County, Florida. Florida Scientist. 47(3):180-187.

Zoodsma, B.J. 1991. Distribution and behavioral ecology of manatees in southeastern Georgia. M.S. Thesis. University of Florida, Gainesville, FL. pp. 202.

Packard, J.M. 1981. Abundance, distribution and feeding habits of manatees (Trichechus manatus) wintering between St. Lucie and Palm Beach Inlets, Florida. Report prepared for USFWS Contract No. 14-16-0004-80-105. pp.142.

Lefebvre, L.W. and Powell, J.A. 1990. Manatee Grazing Impacts on Seagrasses in Hobe Sound and Jupiter Sound in Southeast Florida during the Winter of 1988-1989. Final Report prepared for the Marine Mammal Commission in Fulfillment of Contract T62239152.



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