Bycatches appear to be one of the main causes of man-induced mortality of dolphins and porpoises in the Bay of Biscay. However bycatch estimations for common dolphins vary between 550 (observer-based estimate) and 4700 (stranding-based estimate) per year in the area depending on the methodological choice. In addition to problematic monitoring, mitigation measures as the use of pingers have been inefficient in French waters. Consequently, improving the understanding of interactions between small cetaceans and fisheries remains a challenge. This paper describes how divergent scientific estimations of cetacean bycatches can affect interactions between stakeholders. By using the Actor-Network Theory, we focused on scientific and managerial instruments translating small cetacean bycatches in the Bay of Biscay. According to this social science perspective, instruments are not neutral and play a central role in defining how stakeholders look at an issue. Translating observations into an aggregated number of bycatches is an expression of power relationships and is closely linked to stakeholders’ legitimacy. Similarly, voting a directive on the use of pingers conveys some parties’ perspective on the issue. Stakeholders strategies were shown to be influenced by divergent scientific estimations especially if some stakeholders i.e. fishermen or NGOs are not sufficiently involved in the process. Indeed, two different perspectives of the same phenomenon facilitate the controversy and thus reveal the variety of actors’ perspectives. Integrating the various stakeholders’ viewpoints on small cetacean bycatches is therefore necessary to improve stakeholders’ mobilization and so move towards a shared definition of the issue. To conclude, getting a sharper picture of small cetacean bycatches implies being able to include stakeholders’ strategies and representations of the system. Integrating the various ways to look at small cetacean bycatches could be done through experimentation workshops focusing on instruments.