The equivalent beam angle is a key parameter in echo integration, where it is assumed that targets are uniformly distributed within the sampling volume of the transducer beam. For a horizontally oriented sonar, this assumption is violated if the vertical distribution of fish is non-uniform throughout a sample, potentially causing a substantial bias in estimates of fish abundance or biomass. This paper investigates the magnitude of this bias using observations and simulated data, where in each case the vertical distribution of fish within a limited geographical area is estimated.