Choose your language:


Principal Investigator: Alex Zerbini, NMML, AFSC, NOAA Fisheries, USA; Cascadia Research Collective, USA; Instituto Aqualie, Brazil

Funding year: 2010

Main objectives:


  • To investigate potential sources of bias due to observers missing animals at the surface  (perception bias), animals being underwater as the plane passes (availability bias) and bias in estimation of group size for aerial surveys of franciscanas.

  • To investigate whether factors to improve/correct for estimates of abundance of the species can be developed.

  • To improve range-wide estimates of franciscana abundance.

Main outcomes:


  • Experiments to assess visibility and group size bias indicate that uncorrected franciscana density estimates obtained from aerial surveys can be negatively biased by a factor of 4. For this reason, correction factors are needed when aerial platforms are used to estimate abundance of this species.  

  • Surfacing time recorded from boat- and land-based platforms underestimate the time that franciscana groups are available to be detected from aerial platforms. Therefore, availability bias estimates for aerial surveys should be estimated from dive parameters computed from aerial platforms.

  • The relatively narrow range of some environmental variables recorded in this project precludes a more thorough assessment of the effects of these variables on surfacing time of franciscana dolphins. Therefore, the development of experiments similar to those undertaken here in areas within the range of the species with more heterogeneous environments (especially in regards to water transparency and depth) should be conducted to improve estimation of availability bias for franciscana dolphins.



Zerbini et al. 2011. Assessing bias in abundance estimates from aerial surveys to improve conservation of threatened franciscana dolphins: preliminary results from a survey conducted in southern Brazil. SC/63/SM9. Click here to read. 


Sucunza et al. 2014. Evaluating Factors Affecting Availability of Franciscana Dolphins (Pontoporia blainvillei) from Helicopter Surveys in Southern Brazil. SC/65b/SM18. Click here to read.