NIWA CARRIES OUT fish biomass surveys for the Ministry of Fisheries using acoustic techniques. This involves surveying areas of the ocean with an echo-sounder that sends out pulses of sound. Echoes are received back from schools of fish and these are recorded for later analysis. The total strength of the echo from a school is divided by the strength of an echo from an “average” fish to estimate the number of fish present. Therefore we need to know how much sound is reflected from various species of fish – known as their target strength – and how this varies with fish size and orientation. For further details on how fish biomass is calculated using acoustic surveys, see Water & Atmosphere 4(1): 13–17. This article describes the swimbladder modelling technique for estimating the target strength of fish.