Mass reproduction is widely used in fish farms because it is relatively easy and inexpensive. It also enables the development of mass selection programmes that provide gains for traits with high heritability. The main weakness of this approach is that it is currently impossible to know which females participated in reproductive events, unless by conducting expensive genomic experiments. We tested oviduct-inserted floating passive integrated transponder tags as a new method to detect (i) the identity of spawners and (ii) the timing of spawning. We first conducted a preliminary experiment using four small tanks (1.5 m3) each containing one male and one female tracked by infrared video cameras to test the experimental device. We then tested it in “real” aquaculture conditions, using a bigger tank (10 m3) containing 89 adult fish. Results showed that this tracking system accurately identified the timing of spawning of individual fish. We confirm that European sea bass preferentially spawn at sunrise or sunset. This proof-of-concept developed for one commercially important fish species could also be used for novel species of interest for aquaculture, for example, to determine the exact timing of spawning after hormonal treatment of novel species.