Unlike terrestrial animals, fish determine the direction of sound from acoustic particle motion cues. However, sounds coming from opposite directions generate similar particle motion. Fish possessing gas filled swim bladders are thought to detect sound pressure indirectly via the stimulation of the inner ear otoliths from the sound scattered by the swim bladder. It is believed that pressure detection aids in distinguishing sounds incident from opposite directions. However, few experiments have been conducted to test this hypothesis. Plainfin midshipman have become model organisms to investigate sound source localization because of the innate attraction of females to the advertisement calls produced by type I “singing” males. We are using computerized tomography scans of the inner ear otoliths and the swim bladder in a finite element model to study how the presence of a gas filled swim bladder modifies the motion of the three paired (left and right) otoliths of the saccule, lagena and utricle to low frequency monopole sound sources (within the natural hearing range) in juvenile, adult male (type I), and adult female plainfin midshipman. In our analysis, we varied both the frequency and direction of incident sound relative to the swim bladder. Preliminary investigations indicate that the presence of the swim bladder amplifies the motion of lagenar otoliths in the vertical direction. Since the hair cells corresponding to these otoliths are maximally sensitive to vertical vibrations, the lagena may function as an indirect pressure sensor, detecting sound re-radiated from the swim bladder.