Target strength

The target strength is a logarithmic measure of the area of a sonar target. It is typically used to quantify acoustical size of fish, marine mammals, mines or submarines and is expressed in units of decibels. For fish such as salmon, the target size varies with the length of the fish and a 5 cm fish could have a target strength of about -50 dB.

The target strength of a fish also depends on the orientation of the fish at the moment of sonification, which in turn changes the scattering cross-section of the fish and any air-filled cavities of the fish. The effect of this means that behavioral reaction affects observed biomass, for example fish evading the research vessel at night due to strong lights and vibrations from the hull and machinery. Target strength is often observed on or near a specific frequency where the target is most resonant. Narrowband (CW) pulses have historically been used, but there is ongoing research into using wideband (FM) pulses for improved classification.