Integrated Population Model for Southwest Florida

For the first time, the Florida Fish and Wildlife Conservation Commission (FWC), United States Geological Survey (USGS) and Mote Marine Lab (MML) scientists used a state-of-the-art technique called integrated population modeling (IPM) to reconstruct trends in Florida manatee abundance in southwest Florida. In the past, FWC used counts from aerial surveys that were not adjusted to reflect those missed by the observers (called synoptic surveys), along with educated guesses to make inference about historic trends. The IPM integrates multiple sources of information to obtain more accurate and precise estimates of how manatees in southwest Florida have been doing since 1997. These sources of information include FWC’s abundance estimates from aerial surveys, FWC’s carcass salvage program, and the cross-agency (FWC, MML, USGS) photo-identification program for estimating manatee survival and reproduction.

Using the IPM, scientists estimated that the population in southwest Florida grew from 2,014 manatees in 1997 (the 95% credible interval [CRI] was 1,861–2,229; simplified definition of 95% credible intervals: given the observed data and the model, we are 95% confident that the "true value" lies within the CRI) to 2,966 (CRI: 2,551–3,434) manatees in 2016 for an average growth rate of 2% (CRI: 1–3%). However, the population has not grown steadily (Figure 1). For example, scientists estimated that in 2013 (a year of an intense red tide in southwest Florida) the manatee population decreased by approximately 331 animals (CRI: 217–459).

Figure 1. Southwest Florida annual abundance estimates from IPM, 1997–2016 (IPM posterior) and the two aerial abundance surveys. Symbols indicate estimates; line segments indicate 95% credible intervals. Synoptic survey counts are shown for reference.

The model also can help improve our understanding of environmental effects on manatee population dynamics, which will enhance our ability to make accurate forecasts. For example, the IPM can be used to quantify the impact of the red tide on manatee mortality and abundance. 

Estimates from the IPM can be updated on a periodic basis with data currently being collected by FWC and partner agencies. The model can also be combined with simulated datasets to help plan monitoring activities, including how to allocate resources to various aspects of the monitoring program, such as aerial surveys, photo-identification, genetic sampling, and carcass recovery. This could help inform how often new data needs to be collected, which may result in lowering the costs of monitoring the population.

Although this study currently applies only to the southwest management unit (Figure 2), it is a starting place for scientists to develop similar models across the entire range of this threatened subspecies.

Figure 2. Map of southwest Florida, location of the study population. Florida is divided into four manatee regions or management units (see inset): northwest (NW), southwest (SW), Atlantic coast (ATL), and Upper St. Johns River (USJ). Stars on the map indicate primary locations at which photo-identification data were collected (all are manatee warm-water aggregation sites). Other data streams used in this analysis (carcass data and abundance surveys) were collected throughout potential manatee habitat in southwest Florida and are not pictured.