FWC’s Third Update to the Statewide Florida Manatee Abundance Estimate

The Florida Fish and Wildlife Conservation Commission (FWC) conducted its first statewide Florida manatee abundance survey in 2011 and 2012, publishing the resulting abundance estimates in 2015 (Martin et al 2015). A second abundance survey was conducted in 2015 and 2016, with updated abundance estimates published in 2018 (Hostetler et al 2018). A third abundance survey was completed in 2021 and 2022, and the results of that survey are now available (Gowan et al 2023). These surveys accomplish a primary conservation goal of the FWC’s Manatee Management Plan by implementing “peer-reviewed and statistically sound methods to estimate the manatee population and monitor trends.” The abundance survey is scheduled for a time of year when all Florida manatees should be in the state but are more spread out instead of aggregated at warm water and winter habitats. The survey is an intensive effort that takes place over the course of a week or more for each coast (the two coasts are flown in consecutive years) where we sample thousands of locations that are randomly selected (within guidelines) by a computer. A double observer approach was used to independently count the number of manatees seen by each observer at each location. Our method accounts for important sources of error, such as manatees that are not seen by observers during the survey, and provides a statistically sound framework for estimating statewide manatee abundance. 

Flights for the third statewide Florida manatee abundance survey were conducted December 1–15, 2021 on Florida’s west coast and November 30–December 6, 2022 on the east coast. Our estimate of statewide abundance for the 2021-2022 period is 8,350–11,730 manatees, with 3,960–5,420 on the west coast and 3,940–6,980 on the east coast. These ranges are the 95% Bayesian credible intervals, indicating that we are 95% confident that the true population size lies between the lower and upper bounds if our statistical assumptions are correct. Unlike a single point estimate (such as the mean or median), these credible intervals describe the uncertainty in the abundance estimates. Our credible intervals are broad because of issues like manatees being submerged where they cannot be seen by observers, observers missing manatees that are available to be seen, and observers not being able to sample all locations where manatees could be.  It is important to consider these limitations and others described below when using these numbers to assess the status of the manatee population.

Several points must be considered when interpreting these results: 1) we accounted for several factors that influence the probability of a manatee being detected (such as water visibility and manatee diving behavior), but their effects may not be fully represented in the model; 2) groups of manatees may be more easily detected by observers than individual manatees, but groups and individuals are treated the same way in this model; and 3) we used a random sampling design because it is impractical to comprehensively survey all manatee habitat, but sampling variability means that, by chance, we could have surveyed areas that have either higher or lower manatee density than average, and generalizing those results to unsurveyed areas could  bias estimates of population size.

Our abundance surveys are a large undertaking and are logistically challenging on many levels. They require many trained observers and pilots (up to seven aircraft and 14 observers per day, sometimes on short notice), and the weather needs to be favorable over an entire coastline for more than a week. Because of these challenges, we fly only one coast each year. The abundance estimates are presented on both a statewide and coastal scale. However, biologically, there is little movement between coasts, which supports an east-west division for management purposes.

Although the bounds of the 95% credible interval for the 2021-2022 survey (8,350–11,730) are higher than those from the 2015-2016 survey (7,520–10,280), the intervals overlap, indicating the population could have either increased or decreased since the last survey. Because there is considerable uncertainty in these estimates, we don’t recommend projecting trends from the abundance survey estimates alone. Instead, we are currently implementing an Integrated Population Model (IPM), which is a more robust approach to estimating population trends that makes use of additional information. By combining the abundance survey estimates with survival and reproductive rates estimated through photo-identification and the number of manatee carcasses verified through the mortality response program, the IPM can reduce uncertainty and bias in abundance estimates and provide abundance estimates in years when the survey was not conducted. The IPM has already been applied to Florida manatees in the Southwest management unit (Hostetler et al 2021), and we are currently working to apply this model to other regions of the state. These models will provide a cost-effective means to increase the accuracy of estimates of abundance, population growth rates, and impacts of unusual mortality events on the Florida manatee populations.