Loggerhead turtles have been surveyed off the east coast of the U.S. by plane in order to estimate their seasonal density and abundance. The protocol used by observers in the plane allowed mark-recapture distance sampling (MRDS) methods to be used so that the probability of detection of turtles by the observers could be estimated. Conventional distance sampling methods assume that all animals that are on the centreline of the strip, or transect, being surveyed will be detected; if this assumption is not valid, the probability of detection will be overestimated. A mark-recapture distance sampling (MRDS) approach allows the probability of the detection on the centreline to be estimated. Two observers (or two teams of observers) are required and the numbers of animals seen by one, or both, of the observers are used to estimate this probability.