USING AIRCRAFT IN TANDEM FORMATION TO ESTIMATE ABUNDANCE OF HARBOUR PORPOISE

The use of aircraft for line transect survey of cetaceans allows near-synoptic survey of coastal waters but small pods may be missed along the trackline. This paper describes how accurate recording of pod sightings from two aircraft in tandem formation can be used to estimate the effective strip width (ESW), allowing for the missed pods. The estimate is based on the existence of duplicates (pods seen from both aircraft), but instead of using subjective assessment of which pods are duplicates, the technique considers all candidates among the leading/trailing sighting pairs. The estimated proportion of missed pods is consistent with what is known about the diving behaviour of harbour porpoise. The performance of the technique was checked using simulation trials.