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Foraging
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
Bureau of Protected Species Management
(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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