Improving river dolphin monitoring using aerial surveys

Abstract

AbstractObtaining correct abundance estimates to model population dynamics and promptly adjust management actions is a key goal of threatened species conservation. For elusive aquatic mammals like river dolphins, however, gathering such basic data can be extremely challenging. River dolphins in shallow waters are surveyed from canoes, with the (untested) assumption that visual counts are exhaustive estimates of animal population. These counts, however, should take into consideration that river dolphins usually emerge for very short periods and show very little of their body when surfacing. Recently, aerial survey methods have been introduced as new technology aiming to improve the reliability of wildlife censuses. Herein, we developed and tested a new survey method using a digital camera mounted on a blimp (non‐rigid airship system) for aerial survey of river dolphins. We conducted our experiment by surveying the recently described Araguaian river dolphin (Inia araguaiaensis) during the low water season in the Parque Estadual do Cantão, a seasonally flooded lake system in the Amazonian forest, Brazil. Our survey, which was designed to monitor the river dolphin in 56 survey units within six main survey sectors, implied deploying two canoes navigating simultaneously: the first one with a single operator holding the blimp and recording data from its camera and the second one with three observers performing standard visual counts of dolphins. In contrast to common expectations, visual counts detected on average <75% of the dolphins recorded by the blimp's camera. Compared to the visual survey, the aerial method was able to detect more individuals, more dolphin groups, and to record more calves, which are usually difficult to detect. The differences in performance between the two methods were remarkable when surveyed sectors were larger. We showed that visual counts from canoes are less exhaustive surveys than aerial surveys, and provided a less expensive method (one operator needed to handle the blimp, compared to three visual observers) to count aquatic mammals. Our results can be used to correct current visual count estimates. With further development of blimps and drones in wildlife surveys, we foresee a promising future for the conservation and management of threatened species.