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Assessing bias in aerial surveys for threatened cetaceans: Results from experiments conducted with the Franciscana (Pontoporia blainvillei)
F. Sucunza, D. Danilewicz, E. R. Secchi, A. Andriolo, M. Cremer, P. A. C. Flores, E. Ferreira, L. C. P. de S. Alves, F. R. de Castro, D. P., C. M. Sartori, B. Schulze, P. Denuncio, M. S. Perez, A. N. Zerbini
International Whaling Commission
1. Line transect aerial surveys are widely used for estimating abundance of biological populations, including threatened species. However, estimates obtained with data collected from aircrafts are often underestimated because of visibility bias or bias in estimating group sizes from a fast-moving platform.
2. An assessment of multiple sources of bias in aerial surveys were carried out in southern Brazil by experiments multiple survey platforms (e.g., boats and aircrafts). These studies focused on evaluating visibility bias (perception and availability bias) and potential differences in the estimation of group sizes from different types of platforms used in franciscana (Pontoporia blainvillei) abundance surveys. The ultimate goal was to develop of correction factors to improve accuracy of estimates of density and population size for this threatened dolphin.
3. Estimates of density and group sizes computed from boats were assumed to be unbiased and were compared to estimates of these quantities obtained from an airplane. A correction factor (CF=4.42, CV=0.04) was computed as the ratio of the density estimated by boats 2.99 ind/km2 (CV=0.23) and by the aircraft 0.68 ind/km2 (CV=0.28). Availability of franciscana groups was estimated at 0.39 (SE = 0.009).
4. Visibility bias was substantial and accounted for ~70% of the total bias. Group sizes estimates from the boats were significantly different (~30% larger) than those from the aircraft and accounts for a relatively large proportion of the bias in the aerial survey estimates of density.
5. The correction factor reported above can be used to refine range wide abundance estimates of franciscanas given certain assumptions are met. The lack of observed effects of environmental variables (e.g. depth and water transparency) on franciscana groups availability indicates the potential use of the independent estimated availability bias along all the species range.