Do close range measurements affect the target strength (TS) of fish in horizontal beaming hydroacoustics?

Abstract

Detailed fish target strength (TS) studies allow us to relate their physical and biological variables to their sound behaviour and they potentially improve the accuracy of acoustic assessments. One of the limitations of the transducers that are currently used is their difficulty to precisely estimate TS at close range, either because the acoustic beam may not include the whole fish or because of the near-field effect. When measuring the sound produced by a target (fish), it is recommended that the so-called near-field area, which is generated immediately in front of the target, should be avoided. In horizontal hydroacoustics, where information is obtained within the first 15–20 m from the transducer, avoiding this near-field area may render the majority of the sampled volume useless. This experiment was developed in order to study the horizontal behaviour of the TS with regard to distance. We have studied three different sizes of free-swimming large carps and barbels (≈300, 400 and 500 mm) and we have recorded fish traces at three distances from the transducer, both fulfilling and not fulfilling the requirements of the near-field (6, 9 and 12 m). The results showed no differences in the mean TS obtained for fish at different distances. Factors such as orientation or size showed a greater influence on TS changes. These results are very encouraging and they support the use of hydroacoustics in studies performed at close range, such as studies of fish migration in rivers or biomass estimations in aquaculture.