Second, effort hours are restricted by optimal weather conditions and daylight. First, these species spend most of their time underwater and many are widely distributed and highly mobile. Although abundance data are key, there are difficulties considering when monitoring aquatic mammals should be carried out ( Taylor et al., 2007). Population assessments play a critical role in informing conservationists about the effects of anthropogenic impacts ( Legg & Nagy, 2006). To facilitate effective species conservation, a good understanding of population density is crucial ( Nichols & Williams, 2006 Gómez-Salazar et al., 2012). Keywords: Inia geoffrensis Sotalia fluviatilis boto tucuxi population abundance seasonality echolocation loggers C-POD This work highlights the possibility of monitoring both river dolphin species through visual and acoustic surveys to generate baseline information on abundance trends and distribution patterns in the Ucayali region, an area with high levels of human disturbance. Preliminary acoustic surveys undertaken with C-POD passive acoustic monitoring devices indicated a diel pattern in detections of cetacean vocalization with more diurnal activity and with detections ceasing from midnight until 10:00 AM. Visual surveys observed more Sotalia individuals during earlier hours of the day and during the dry season. Average density for Inia was 0.2 ± 1.2 ind km −2 with overall abundance estimated at 3 individuals (95% CI: 0 to 8). Based upon total survey effort, we estimated Sotalia had a mean density ± SE of 1.98 ± 4.6 ind km −2 and an overall abundance of 34 individuals (95% CI: 28 to 40). A total of 324 Sotalia observations and 44 Inia observations were recorded in boat-based transects. We present results of the first simultaneous visual and acoustic surveys for Amazon River dolphins ( Inia geoffrensis) and tucuxi ( Sotalia fluviatilis), undertaken in Yarinacocha Lagoon, Ucayali, Peru (length = 20 km, area = 12 km 2).
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