An anatomically detailed acoustic scattering model has been used to estimate the backscattered target strength (TS) of six mackerel icefish (Champsocephalus gunnari) of total length 26.8–35.0 cm, at 38 kHz. The model used computed tomography (CT) scans of fish to determine the sound speed and density throughout the fish and simulated the complex interaction of the incident sound wave with the acoustic impedance contrasts throughout the fish. A preliminary length to tilt-averaged TS relationship is derived from the model results: 148)(log4.7010-=LTSwhere L is the fish total length in cm and is applicable to fish of length 26–35 cm. Existing in situ TS estimates are for smaller fish (16–26 cm) and hence are not directly comparable, but extrapolation of the in situ and model results reveal a large different in TS (6 dB at 26 cm) between the two datasets.