The abundance of manatees, as with most marine mammals, is difficult to determine because they are visible for only short periods of time while at the surface of the water (Eberhardt et al. 1979, Powell et al. 1981). Aerial surveys are generally considered to be the most accurate method of counting manatees, although there is no doubt that some undetermined proportion is missed (Irvine and Campbell 1978, Shane 1981, Kinnaird 1985, Packard et al. 1985). When designing aerial censuses for manatees two types of aircraft are usually considered: fixed-wing and rotary-wing airplanes (Irvine 1982). Fixed-wing airplanes normally are used because helicopters are prohibitively expensive to hire (often over twice that of a fixed-wing airplane). Helicopters, however, have been assumed to yield more accurate counts for manatees (personal observations; J. Fletemeyer and J. Provancha, personal communication) because their slower air speed and greater maneuverability reduce the proportion of animals missed due to some of the biases inherent in fixed-wing airplane counts (Caughley 1974). To assess how effective fixed-wing aircraft are in determining the number of manatees in an area, a series of paired total-count aerial surveys were done in winter 1978-79.

A Bell 47G helicopter was used to obtain a “count” to compare with counts made from a Cessna 172 fixed, high-wing airplane. The experiment was conducted at two different sites and on five consecutive days at each site. The first location was Crystal River, which is a clear, spring-fed river that flows about11 km into the Gulf of Mexico on the central west coast of Florida. The second site was an 8-km-long section of the Indian River, which is a large, turbid estuary adjacent to Cape Canaveral on the east coast of Florida.