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 estimat e population size of manatees (Trichechus manatus) in the Banana River, Flo rida, 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 indep endent observers, and (3) to evaluate the usefulness of these surveys in de tecting population trends over time. Five hundred thirty-one individuals co mprising 248 manatee groups were counted in 15 replicate surveys during Aug ust-September 1993 and August 1994. Survey-specific correction factors for perception bias (the proportion of manatees visible within the strip transe ct but missed by observers) averaged 1.12. Annual population estimates deri ved 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 str atum (i.e., manatee aggregation sites) to obtain corrected population estim ates 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 y ears with power greater than or equal to 0.75. Our study is the first to us e replicated strip-transect aerial surveys to estimate manatee population s ize. This survey technique is an improvement over previous attempts to esti mate manatee abundance, because it uses a repeatable, standardized survey d esign to produce population estimates with associated variance and confiden ce intervals. We recommend that managers use warm-season strip-transect sur veys in the Banana River to corroborate other evidence of population trends of manatees on the east coast of Florida.