Using calls to estimate the abundance of inshore dolphins: a case study with Pacific humpback dolphins Sousa chinensis

1 Assessing the number of animals in a population is a fundamental requirement for effective wildlife management. Determining this information for cetaceans can be logistically difficult, and the abundance of inshore cetaceans along most of the world's coastline is unknown. 2 In this study we illustrate the potential of using sound as a tool for estimating the abundance of inshore cetaceans, using Pacific humpback dolphins Sousa chinensis . 3 Acoustic recordings of humpback dolphins were made in conjunction with visual observations of school size from a land-based platform on Stradbroke Island, Australia. 4 The mean number of calls recorded per 3-min sample period was regressed against the number of dolphins observed using schools for which more than three sample periods were recorded. The relationship estimated number of dolphins = 2·39 × the mean number of calls per 3-min sample was used to estimate the number of dolphins in schools for which fewer than four samples were recorded. Comparing these results against known group sizes indicated that this estimation technique is acceptable. 5 Recordings could be made using remotely deployed hydrophone units or submerged autonomous units to provide information on the occurrence and group size of inshore delphinids within an area. 6 Inexperienced personnel could deploy and retrieve recording units. Analysis would require training to recognize sound types from different species, or computer-based sound recognition programs. 7 We conclude that simple techniques using phonations can provide estimates of relative abundance for one species of inshore cetacean. The technique is acceptable for groups of fewer than nine individuals and should be developed to assess its usefulness for studying other species.