The effect of behaviour on target strength is simulated through computation of the average target strength with respect to a range of normal orientation distributions. The mean tilt angle is varied from -30 to 30 deg for each of two values of the standard deviation 2 and 5 deg. The object fish are assumed to be uniformly distributed over the beam of a transducer with effective 10-deg vertex angle. The basic data consist of measurements of the tilt angle dependence of target strength of immobilized and tethered fish. Average target strengths are regressed on the logarithm of fish length. Several statistical tests are performed to determine the similarity of the length coefficiens to 20 and similarity of regressions for difference species.