The invention relates to an identification chip for insertion in an object located in a liquid. The identification chip comprises an acoustic resonator (100), which exhibits a number of distinct resonant frequencies, where the combination of resonant frequencies is unique to the identification chip. This enables the identification chip to be identified by exposing it to an acoustic polling signal, measuring an acoustic response signal and analysing the frequency of the response signal. The resonator (100) comprises a cavity-forming part (110, 120) and a membrane (130). The acoustic resonant frequencies are determined by at least one cavity (140), which is enclosed by the cavity-forming part (110, 120) and the membrane (140). The identification chip can be implanted in a fish, with the object of identifying the fish when it is located in water.