A double-platform protocol was implemented in the Bay of Biscay and English Channel during SCANS-III survey (2016). Two observation platforms using different protocols were operating on-board a single aircraft: the reference platform ("Scans"), targeting cetaceans, and the "Megafauna" platform, recording all the marine fauna visible at the sea surface (jellyfishes to seabirds). We tested for a potential bias in small cetacean detection and density estimation when recording all marine fauna. At a small temporal scale (30 seconds, roughly 1.5 km), our results provided overall similar perception probabilities for both platforms. Small cetacean perception was higher following the detection of another cetacean within the previous 30 seconds in both platforms. The only prior target that decreased small cetacean perception during the subsequent 30 seconds was seabirds, in the Megafauna platform. However, at a larger scale (study area), this small-scale perception bias had no effect on the density estimates, which were similar for the two protocols. As a result, there was no evidence of lower performance regarding small cetacean population monitoring for the multitarget protocol in our study area. Because our study area was characterised by moderate cetacean densities and small spatial overlap of cetaceans and seabirds, any extrapolation to other areas or time requires caution. Nonetheless, by permitting the collection of cost-effective quantitative data for marine fauna, anthropogenic activities and marine litter at the sea surface, the multi-target protocol is valuable for optimizing logistical and financial resources to efficiently monitor biodiversity and study community ecology.

Data_for_Lambert_et_al_2019_RSOSData and codes used to conduct analyses presented in the paper Lambert et al 2019 "The effect of a multi-target protocol on cetacean detection and abundance estimation in aerial surveys".