Detection probability

The detection probability is the likelihood of detecting echoes from individual organisms. Single target detection probability is dependent on the imposed thresholds and the behavior of the organisms. Organism orientation strongly affects TS, as does the vertical distribution. Organisms on the edge of the beam will have lower detection probabilities due to the acoustic beam pattern. The echo level from a fish located at the half intensity beam angle (-3 dB) will be 6 dB lower than if located in the center of the beam (-3 dB for transmitting and 3 dB for receiving the signal in that direction). Organisms near the bottom will have lower detection probabilities due to the bottom dead zone (see below). Organisms close to the surface will not be detected if they are above the depth of the transducer and will give unstable returns if in the near-field. Near surface fish may also have higher avoidance reactions to the survey vessel (see below).

Uncertainty in detection probabilities of single fish affects interpretation of S_v measurements and the efficacy of post-processing techniques. Systematic and random changes in detection probabilities during the survey will have linear and non-linear effects on S_v measurements. A systematic change in fish orientation, for example, from a horizontal to a more vertical position during vertical migration, will cause a decrease in S_v. If factors such as orientation are not taken into account, it might appear that there are fewer or smaller fish. For that reason, surveys should avoid periods of vertical migration.  This is less problematic when using in situ TS to scale Sv values.  However, more work on this topic is needed.