Modeling manatee movements to predict and assess the impact of hydrologic change in estuaries of southwestern Florida

Brad Stith¹, Dean Easton², Jim Reid², Lynn W. Lefebvre², and Don DeAngelis³

1. AScI Corp. under contract for U.S.G.S
2. U.S. Geological Survey, Florida Integrated Science Center,
   Sirenia Project, 412 NE 16^th Ave., Room 250, Gainesville,  FL  32601
3. University of Miami, Miami,  FL

ABSTRACT

We are developing a spatially explicit, individual-based model of the Florida manatee (Trichechus manatus latirostris) in southwestern Florida. This model is part of the Across Trophic Level System Simulation program (ATLSS) that will be used to predict and assess the potential effects of altered hydrologic regimes on manatees in South Florida.  The model uses GIS map layers such as water depth, sea grass beds, fresh water sources, and water temperature to influence the behavior of simulated manatees. We are analyzing telemetry data from GPS and ARGOS tags to help calibrate the model with movement parameters, habitat preferences, time budgets, and behavioral rules. Our current focus is on manatees using the Ten Thousand Islands and nearby areas that will be affected by the restoration of S. Golden Gate Estates.

Study Areas

• Develop a spatially explicit, individual-based manatee model.
• Incorporate detailed GIS layers such as water depth, submerged aquatic vegetation, freshwater, and water temperature.
• Parameterize model with a combination of detailed (15 min.) GPS telemetry data and coarser (6 hour) Argos data.
• Use the model to predict changes in manatee abundance and distribution associated with different hydrologic restoration scenarios.

How much time do manatees spend in different habitats?

How do manatees move among habitats?

How directional are manatee movements?

Model Structure

Behavioral Rules

• Avoidance of water deeper than 6 meters.
• Frequent movement (3 - 7 day intervals) between foraging areas and sites with drinking water.
• Foraging concentrated in offshore seagrass beds and inshore bays with healthy SAV.
• Cool season movement into inshore thermal refugia.
• Strong directionality in movement.

Memory Maps

• Each individual has a unique network of preferred feeding, resting, drinking, and thermoregulating sites.  These sites are connected by travel corridors.
• Maternal transmission of memory maps to offspring.
• Repeat visits to favored foraging areas to feed on new vegetative regrowth.
• Preferential use of select travel corridors.
• Inter-annual site fidelity to inshore thermal refugia during cool season.
• Occasional exploratory forays which add new sites to memory map.

Network map of all known travel corridors connecting foraging, resting, drinking areas and thermal refugia.

A Virtual Manatee Brain

Brain image courtesy of U. Florida,
Wally Welker, Roger Reep, J.I. Johnson

Demographic Parameters

• Mortality and fecundity of different age-stage classes will be incorporated into the model to simulate population dynamics.
• Information on vital rates for southwest Florida will be supplemented with estimates from better-studied areas.
• Environmental stochasticity due to prolonged cold spells will affect mortality.
• Catastrophic mortality due to red tides and hurricanes will be investigated.

Validation Methods

• Face validation by biologists viewing real-time map displays.
• Comparison of real and simulated coarse-scale seasonal home range estimates.
• Comparison of real and simulated fine-scale foraging movements.
• Comparison of aerial survey distribution data with snap-shot of simulated population.
• Comparison of population trend analysis based on transect surveys and demographic modeling.

Alternative Restoration Scenarios to Run

• No restoration.
• Restoration with seasonally pulsed water input at a few point sources.
• Restoration with seasonally pulsed water input at multiple point sources.
• Restoration with even sheet flow over broad areas.
• Drought years.
• High rainfall years.

Acknowledgements

Support for this project was provided by USGS Place-Based Studies program and the Marine Mammal Commission. Logistical support and data were provided by Ten Thousand Island National Wildlife Refuge, Everglades National Park, Rookery Bay National Estuarine Research Reserve, and Florida Marine Research Institute.