Non-native species are one of the leading causes of global change, causing enormous and irreversible damages to ecosystems. Yet, the rate of arrival of new biota continues to accelerate, being particularly detrimental in freshwater ecosystems. Fishes are the most introduced vertebrate group across the globe, being mainly introduced in freshwater ecosystems. However, invasions in aquatic environments generally occur? far from our sight?, being difficult to detect species in early stages of invasion. This hampers an efficient management of invasive species. Interestingly, more than 800 fish species are known to produce sounds, some of which have been introduced in freshwater ecosystems. In this project, we aim to evaluate the use of passive acoustics to detect invasive fish and therefore its potential use as tool to early invasion stages, focusing on two non-native fishes that recently arrived to the Tagus River and still have a restricted distribution.