Estimating food consumption of free-ranging manatees in Florida [Trichechus manatus]

The quantity of aquatic plants consumed by manatees (Trichechus manatus ) is of interest from an energetic, an ecological, and, as well, a management perspective. Agencies concerned with aquatic plant growth in Florida need information about the food requirements of manatees to properly manage habitat. From January 1979 through December 1980, the ecology of manatees was studied at Blue Spring Run, Volusia County, and along the St. Johns River in Florida. As part of the study, the author estimated manatee food consumption under natural conditions by developing predictive curves relating the number of chews required for a manatee to consume a certain weight of vegetation. The sounds generated by manatees chewing vegetation underwater were monitored with a hydrophone (R-130, Inter-Oceans Systems, Inc., San Diego, Calif.) attached to a headset. These crunching noises were counted in the field. If the correlation between plant weight and number of chews was high, the plant weight consumed could be estimated by counting the number of times a free-ranging manatee chewed during a feeding bout. Three sets of data were required to use this technique: feeding curves showing the relationship between the number of chews needed to eat a given weight of plant material, counts of chews per unit time of manatees feeding under natural conditions, and time budget data identifying the amount of time spent feeding.