An evaluation of strip-transect aerial survey methods for monitoring manatee populations in Florida

We evaluated the use of replicated strip-transect aerial surveys to estimate population size of manatees (Trichechus manatus) in the Banana River, Florida, an important warm-season refuge for this species. Our objectives were (1) to estimate population size of manatees, (2) to correct for perception bias by applying a Petersen mark-recapture model to counts made by 2 independent observers, and (3) to evaluate the usefulness of these surveys in detecting population trends over time. Five hundred thirty-one individuals comprising 248 manatee groups were counted in 15 replicate surveys during August-September 1993 and August 1994. Survey-specific correction factors for perception bias (the proportion of manatees visible within the strip transect but missed by observers) averaged 1.12. Annual population estimates derived from strip transects were 125 +/- 4 ((x) over bar +/- SE) and 179 +/- 8 in 1993 and 1994, respectively. We added counts made in a high-density stratum (i.e., manatee aggregation sites) to obtain corrected population estimates of 159 +/- 7 and 238 +/- 10 in 1993 and 1994, respectively. Given the high precision of our annual estimates (CV less than or equal to 0.05), we could use this survey protocol to detect a 5% annual rate of change in <4 years with power greater than or equal to 0.75. Our study is the first to use replicated strip-transect aerial surveys to estimate manatee population size. This survey technique is an improvement over previous attempts to estimate manatee abundance, because it uses a repeatable, standardized survey design to produce population estimates with associated variance and confidence intervals. We recommend that managers use warm-season strip-transect surveys in the Banana River to corroborate other evidence of population trends of manatees on the east coast of Florida.