Freshwater cetaceans are one of the most threatened groups of mammals on earth. Limited resources for monitoring and low power to detect trends hinder the development of effective conservation. Using the southern Bangladesh subpopulation of Ganges River dolphins (Platanista gangetica gangetica), previously thought to be a closed population, I investigate cost-effective boat-based methods for monitoring and estimating population size, and the value of local informant data for contributing to knowledge on the status of this poorly-known subpopulation. Detectability must be accounted for during surveys to make inferences on species’ trends. However, many surveys use methods that do not account for detectability, assuming such approaches to be cheaper. I demonstrate that a combined visual-acoustic survey is a robust and cost-effective approach for monitoring. Using data from multiple seasons and marine surveys, I show the population may not be closed. I develop correction factors to account for imperfect detectability during past visual-only surveys and use these to show there is no detectable long-term (1999-2012) change in the abundance of this subpopulation. Local informant data are sometimes considered to have the potential to provide information of value to monitoring population trends. A comparison of the long-term and seasonal trends from boat-based surveys and those reported by fishers showed poor agreement. Memory-related biases are likely to have impacted informant recall. However, local informant data proved useful in identifying causal mechanisms underlying dolphin susceptibility to bycatch in gillnets, in particular river depth and net mesh size. Furthermore, local informant data provided a minimum estimate of annual mortality that is deemed unsustainable, but is based on a number of assumptions and potential biases that are discussed. Combined visual-acoustic surveys and local informant data represent cost-effective tools for addressing some of the significant knowledge gaps on freshwater cetacean status, aiding the development of evidence-based conservation strategies.