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.C.; FERREIRA, E.; ALVES, L.; SUCUNZA, F.; DE CASTRO, F.; PRETTO, D.; SARTORI, C.; SCHULZE, B.; DENUNCIO, P.; LAAKE, J.

2011-02-14/2011-03-01

1-13

Biológica

Otros campos

Aerial surveys are considered the most effective way of estimating abundance of franciscanas (Pontoporia blainvillei). 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. Independent boat and aerial surveys were concurrently carried out in Babitonga Bay, southern Brazil, to assess potential bias in aerial surveys for franciscana dolphins. Estimates of density and group sizes from the boats were assumed to be accurate (i.e. not affected by visibility or perception bias) and a preliminary correction factor (CF=4.74, CV=0.05) was computed as the ration 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 different (30% larger) than those from the aircraft and accounts for some of the bias in the aerial survey estimates. Visibility bias was substantial and accounted for 70% of the total bias. The correction factor reported above can be used to refine rangewide abundance estimates of franciscanas given certain assumptions are met. Additional work is underway to further refine the analysis presented in this document and results will be made available in the future.