Coastal waters off Nouakchott support fisheries and, more recently, offshore oil activity and iron-ore export, all of which raise the risk of marine contamination. Local biomonitoring tools are still lacking. We asked whether the wedge clam Donax rugosus, abundant on Nouakchott’s beaches, can serve as a sentinel. Acetylcholinesterase (AChE), an enzyme inhibited by organophosphate and carbamate pesticides and by several trace metals, was measured together with the physiological condition index (CI). Clams were collected monthly from September 2022 to July 2023 at two beaches, Nicola and Hôtel Sabah. AChE was first assayed in four organs (mantle, digestive gland, foot, gill) and then followed monthly in the gills. Activity differed strongly among organs: the mantle was highest (about 16.0 nmol min⁻¹ mg⁻¹ protein, two-site mean) and the gill lowest (about 2.6), roughly a six-fold range. We retained the gill for the time series because its low, stable baseline is easy to read. Gill AChE and CI both changed with season, each peaking in summer; gill AChE was lowest in autumn and rose with water temperature, while CI fell in spring, the period when Donax spawns. Paired tests showed no significant difference between the two beaches for either variable, and the AChE–CI link was weak overall. Gill AChE in D. rugosus is a workable, low-baseline biomarker for coastal biomonitoring in Mauritania, on the condition that temperature and reproductive state are accounted for when the data are interpreted.