We conducted a study to assess the populations of the two potentially endangered river dolphin species Inia geoffrensis and Sotalia fluviatilis in the river systems of the Río Tigre and the Río Marañon in the Peruvian Amazon region. Various observation methods were used and evaluated along ca. 353 kilometres of river and 134 to a maximum of 173 dolphins were recorded from 125 sightings and about 2000 photographs. Only about 10% of these dolphins could not be determined to species level, at least 57 individuals were identified as S. fluviatilis, and 89 individuals as I. geoffrensis. The abundance of dolphins along the entire stretch was 0.13 individuals per km for S. fluviatilis and 0.25 individuals per km for I. geoffrensis. Each participant spent an average of 82 working hours on observation. Evaluation of methods used revealed that observation supported by photographic documentation (PSO) increases observation accuracy by 19%. PSO is thus regarded as an ideal method for future population assessments. Perspectives and potential financial costs for conservation of Amazonian dolphin populations are discussed.

Animal conservation, aquatic mammals, dolphin numbers, dolphin research, endangered species, river dolphin populations, pink river dolphin, population size, whale watching.