Regional variability in feeding habits of the Florida manatee (Trichechus manatus latirostris): a preliminary assessment using ¹⁵N and ¹³C

Abstract

In an attempt to assess the effectiveness of stable isotope analysis to determine diet composition of wild manatees, twenty-five species of vegetation commonly consumed by manatees were collected from four regions in Florida, analyzed for relative values of stable carbon (¹³C) and nitrogen (¹⁵N) using a mass spectrometer, and compared to stable carbon and nitrogen values of epidermal, dermal, and muscle tissue from manatee carcasses (n=25) recovered in the same four regions. Relative abundance of stable isotopes of carbon and nitrogen was also measured for sloughed skin collected from two manatees being rehabilitated in captivity prior to re-release and dermal tissue from one manatee that died of natural causes while in captivity. Relative values of isotopes of ¹³C ranged from -8.7[0/00] (±0.2) to -28.3[0/00] (± 0.1), relative values of ¹⁵N ranged from -0.8[0/00] (±1.0) to 6.4[0/00] (±0.0). The C₄ grass Spartina alterniflora had an average []¹³C value of -13.5[0/00] (±0.2) and []¹⁵N of 3.6[0/00] (±2.4). Seagrass species that were sampled had values ranging from -8.7[0/00] to -16.9[0/00] for []¹³C and 2.8[0/00] to -2.5[0/00] for []¹⁵N. There was no statistically significant difference in isotope values of manatee tissue types or plant components. Even so, some individuals did exhibit substantial variation between dermal layers, possibly indicating food and habitat changes within a period less than or equal to the half-life of elemental turnover time for the tissues examined. A mixing model was applied to estimate and compare diet composition by region. Results of the model indicated that manatees in all regions consumed 44% or more of their diet from marine and/or estuarine sources. Manatees in the Brevard County area obtained 100% of their diet from marine and/or estuarine sources. Results suggest that stable isotope analysis may be a viable tool to further research manatee food habits. When used in conjunction with telemetry and observational data stable isotope data may aid in the identification of habitat critical to the continued survival of this species. Results of this research suggest marine and estuarine environments are important sources for manatees though where freshwater vegetation is available results suggest it is utilized.