Following a controversy over the construction of a waste incinerator in the Fos-sur-Mer industrial area, residents pointed to the lack of knowledge of the industry’s cumulative impact on their health and environment. Under pressure, some of their elected representatives supported the creation of an independent scientific organization, the Ecocitizen Institute for Pollution Awareness. Its objective was to conduct localized scientific research on the effects of pollution and to lobby the administration to change its regulatory practices. This paper examines the efforts (...) made to ensure that the “undone science” gets done, by focusing on the specificities of this industrialized site. We look at a participatory biomonitoring experiment that aimed to document pollution in the Gulf of Fos where scientists working for the IECP accepted anglers’ requests and switched from an acknowledged sentinel species to another species. We tell the many stories that were shared with us about how conger qualified as a more suitable “cosmopolitical fish” in the study of pollution. Elaborating on actor–network theory and multispecies ethnographies, we discuss the appropriateness of congers as the newly appointed sentinel species. We argue that this demonstrates the importance of the “ecology of relations” in maintaining the livability of the area. (shrink)