Searching for trends in river dolphin abundance: designing surveys for looming threats, and evidence for opposing trends of two species in the Colombian Amazon

Rivers worldwide, particularly in tropical regions, support multiple human uses that can threaten water security and cause species decline. Some tropical rivers are home to obligate freshwater cetaceans (river dolphins) that are vulnerable to exploitation and to upstream and downstream habitat degradation due to limited dispersal opportunities. Assessing vulnerability is complicated by difficulty in reliably estimating abundance, due to the animals' cryptic nature and complex, dynamic habitat. We compared density estimates from surveys conducted in 1993, 2002 and 2007. Surveys were not part of a coordinated monitoring plan and thus were conducted using slightly different methods in different months, which complicated statistical inference. We used information from the most recent survey to account for bias and uncertainty in earlier estimates and used bootstrap and Bayesian approaches to estimate trends, conditional on a plausible range of process variance associated with seasonal movements. For Inia, probability of decline was > 0.75, even under the highest seasonal movement levels considered. For Sotalia, there was a > 0.75 probability of population increase. There are 151 proposals pending in the Amazon for large (> 2 MW) hydroelectric developments that would fragment habitat, and reports suggest that Inia is experiencing illegal killing for fish bait. In this context, our population trend estimates are cause for concern, but improved monitoring is needed to more reliably assess population status. Based on lessons learned from our analysis, future surveys will be standardized in terms of timing (conducted during the transitional water season) and methodology (using our most recent field protocols) to minimize confounding factors and provide more robust inference about population trends. We provide recommendations for ways to distinguish seasonal movements from annual population trends to guide Amazon river dolphin conservation. Until then, two interpretations exist: either Inia is declining, or existing information cannot detect declines unambiguously without additional surveys. Neither explanation bodes well, given the myriad anthropogenic stressors Amazon river dolphins face.