Assessing bias in abundance estimates from aerial surveys to improve conservation of threatened franciscana dolphins: preliminary results from a survey conducted in southern Brazil

ZERVINI, A.; DANILEWICZ, D.; SECCHI, E.; ANDRIOLO, A.; CREMER, M.; FLORES, P.A.; FERREIRA, E.; ALVES, L.; SUCUNZA, F.; DE CASTRO, F.; PRETO, D.; SARTORI, C.; SCHULZE, B.; DENUNCIO, P.; LAAKE, J.

Cartagena de Indias

Congreso; XVI Reunión de Trabajo de Especialistas de Mamíferos Acuáticos de América del Sur; 2014

Asociación Colombiana de Zoología

Aerial surveys are considered an effective way of estimating abundance of the most threatened cetacean in South America: the franciscana (Pontoporia blainvillei). However, estimates obtained with data collected from fixed-wing aircrafts are often underestimated because of (1) visibility bias or (2) bias in estimating group sizes from a fast-moving platform. In February 2011, independent boat and aerial surveys were concurrently carried out in Babitonga Bay, Santa Catarina State, Brazil, to evaluate potential bias in aerial surveys for franciscana dolphins. This study was funded by the International Whaling Commission (IWC) Small Cetacean Conservation Fund. Estimates of density and group sizes from the boats were assumed to be accurate (i.e. not affected by availability or perception bias) and a correction factor (CF=4.74, CV=0.05) was computed as the ratio of the density estimated by boats (D=3.32 ind/km2, CV=0.22) and by the airplane (D=0.70 ind/km2, CV=0.26). Group sizes estimates from the boats were significantly larger (by nearly 30%) than those from the aircraft and accounted for some of the bias in the aerial survey estimates. Visibility bias was also substantial and accounted for ~70% of the total bias. This study provided a quantitative measure of the degree of underestimation resulting from franciscana aerial surveys. The correction factor reported above can be used to refine range-wide abundance estimates given certain assumptions are met and is, therefore, important to improve conservation efforts for this species.