Abundance estimates of franciscana dolphins (Pontoporia blainvillei): a review and future recommendations

Abstract

Surveys to estimate the abundance of franciscanas started in the mid-1990s and early 2000s. The first survey to estimate franciscana abundance was conducted in southern RS, Brazil in 1996. Aerial surveys sampled a broad geographical range within the range of FMA IV, including offshore habitats off the Argentinian Provinces of Buenos Aires and Rio Negro, including Golfo San Matias, which is considered the southern limit of the franciscana distribution. Improvements in survey design included more even sampling effort and greater sampling cover-age of the area for which estimates of abundance were desired. Independent boat and aerial surveys were concurrently carried out in southern Brazil to assess potential bias in aerial surveys for franciscanas. A correction factor for visibility and group size bias for aerial surveys was developed as the ratio of the density estimated by boats and by the aircraft. In addition, in this study, generalized linear models were used to assess differences in group sizes estimated from the boats and the airplane and indicated that boat group size estimates are 33% greater than those computed from an airplane. This suggests that abundance estimates that are not corrected from group size bias, are negatively biased.