The seawater sound velocity and absorption attenuation used in the TS calculation were obtained by measuring the water temperature and salinity values during the experiment. The calibration sphere was suspended approximately 2–3 m below the transducer, and the transmission/reception coefficient (K TR: product of voltage sensitivity and gain) was calibrated by comparing the measured TS to the theoretical TS. Accordingly, a method of measuring TS while allowing the sandeel to swim freely in a relatively large, physically controlled environment is needed.īefore the start of each experiment, the echo sounder was calibrated using a tungstencarbide sphere (φ38.1 mm), which has a standard acoustic reflection with a known TS. Therefore, to accurately estimate the abundance of sandeel, it is necessary to estimate TS in conditions close to their natural swimming conditions, while considering various swimming behaviors. ![]() Since the TS was measured in a fixed space and from multiple individuals, it was difficult to detect individual targets and the fish behavior was restricted. Previous studies of swimming sandeel measured multiple individuals in cages in a tank or in cages under natural conditions. However, it is extremely difficult to measure swimming sandeel because it has schooling behavior and presents a weak acoustic target. Thus, to make good and valid TS measurements on the sandeel, which is a swim-bladderless fish, a method of measuring TS while allowing the sandeel to swim in near-natural conditions is required. Generally, the TS values of fish are influenced significantly by having or not having a swim bladder, and measurements of swim-bladderless fish are more affected by body length and swimming behavior. The in situ method can obtain TS in the natural state however, the inclusion of other fish species introduces large TS errors because TS often cannot directly reveal the target fish measured. The ex situ methods and modeling methods control the tilt of the experimental species and cannot easily obtain TS data similar to those obtained in the natural state. Three methods are commonly used for TS estimation, ex situ methods and modeling methods for live fish in cages and immobile fish, and in situ methods for wild fish. Hence, there is a critical need for the adaptive management of the current sandeel stock. However, the catch fluctuation of sandeel has been on a downward trend this may be due to a decrease in the recruitment of sandeel owing a decrease in the number of adult sandeel, which is strongly affected by marine environmental factors. In Japan, sandeel is widely distributed in coastal areas and is a vital target resource for coastal fisheries nationwide. Therefore, sandeel is an important mid-trophic-level species in the marine ecosystem. In many sea areas, the sandeel occupies a critical ecological position in connecting lower trophic levels to higher ones, because it feeds on zooplankton and is a major prey item of marine mammals. Therefore, it is difficult to identify individual species from catches, and Japan manages these three species of sandeels ( Ammodytes spp.) as sandeel species (hereafter referred to as sandeel) in some areas, such as Hokkaido. ![]() Three species of sandeels ( Ammodytes japonicas Ammodytes hexapterus Ammodytes heian) inhabiting the area around the Japanese archipelago have overlapping distribution areas and external morphologies. The use of such frequency characteristics could facilitate the discrimination of fish species in the field and the sustainable assessment of sandeel species stocks. Furthermore, TS was stronger at 38 kHz than at 120 kHz for all FL classes. In addition, the mean and standard deviation of the swimming angle for the sandeel species from the acoustic data at 120 kHz were −2.2° and 7.7°, respectively, and most were in the −30° to 30°, range. The TS of the sandeel species were not proportional to the square of the FL but increased relatively rapidly with increasing body size. ![]() Parameters a and b used in the estimated TS mean–fork length ( FL: 13.5–21.5 cm) equation were 53.7 and −124.3 dB at 38 kHz, and 71.3 and −153.2 dB at 120 kHz, respectively. In this study, the target strength ( TS) of sandeel species was measured in free-swimming specimens using a split-beam quantitative echo sounder with 38 and 120 kHz frequencies in a physically controlled environment. However, they are strongly affected by the marine environment and their catch rates are trending downward. The sandeel species ( Ammodytes spp.) occupy a critical ecological position in connecting lower trophic levels to higher ones.
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