Horizontal target strength of Luciobarbus sp. in ex situ experiments: Testing differences by aspect angle, pulse length and beam position

Abstract

Horizontal applications of hydroacoustics are used to estimate the density and biomass of fish populations in shallow aquatic ecosystems. In order to calculate fish biomass and density, it is necessary to know the relationship between the biological parameters, such as the length or weight of the studied specimens and the associated target strength (TS). This study presents the results of an ex situ experiment performed on free-swimming specimens of Luciobarbus sp., one of the most diversified and widely distributed genus of the family Cyprinidae. The study was performed using a SIMRAD EK60 split beam echo sounder at 200kHz. TS–length (TS–TL) and TS–weight (TS–W) relationships were derived in the three main orientations: lateral, oblique and head–tail. This study uses a new method in order to calculate the fish aspect angle. This method takes all backscattered fish energy into account and it summarizes the fish movement through regression lines. The TS–TL relationship for the barbel lateral aspect was TSlateral=25.03logTL−99.4 and its results were different from those obtained with other previously published general equations. Thus, the use of this specific equation is recommended in order to obtain accurate estimates of density and biomass in aquatic systems where the barbel is an important component of the fish community. Moreover, the effect of pulse duration on the estimated TS was studied. The results showed no differences between the mean TS obtained by using 0.128ms or 0.256ms of pulse length. Additionally, the influence that the location of the track in the beam has on mean TS was also studied. The results showed that this location does not have a significant influence on fish TS within the first −5dB (one-way) of the beam pattern. This result provides us with a powerful tool to increase the amount of data in TS studies with free-swimming fish where the acquisition process is difficult.