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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