Aerial videogrammetry from a tethered airship to assess manatee life-stagestructure

Aerial videogrammetry from an airship tethered to a boat was used to assess the life-stage structure of manatees in the Blue Waters area of the Homosa ssa River on the west coast of central Florida. Individual frames of videos were loaded onto a computer and lengths of manatees measured using graphic s software. All images of manatees were measured independently by three obs ervers. Measurements were transformed to total manatee lengths by calculati ng the ratio between the measurements of a reference object of known length and the measurement of the manatee. Length estimates were highly variable but were unbiased with respect to observer. Results of an analysis of varia nce suggested that the hypothesis that lengths differed among days and amon g observers should not be rejected. Life-stage structures were represented in three schemes. One scheme-which included four life stages: dependent cal ves, juveniles, animals in tramsition between juvenile and adult, and adult s-was included to provide the most information without sacrificing confiden ce in the life-stage distribution. Overall, the video system was effective at capturing images of manatees for life-stage characterization and, with i mprovements in image resolution, could become a valuable tool for photoiden tification in sight-resight experiments.