Abstract
The body tilt angle of a fish has a large effect on the acoustic target strength. For an accurate estimation of fish abundance using acoustic methods, it is necessary to measure body tilt angles in free-ranging fish. We measured diurnal body tilt angle distributions of threeline grunt (Parapristipoma trilineatum) while swimming in schools in a fish cage. Micro-acceleration data loggers were used to record (for 3 days) swaying and surging accelerations (at 16 Hz intervals) of 10 individuals among 20 forming a school in a fish cage. Time series analysis of 1-h mean body tilt angles revealed significant differences in body tilt angles between day (−7.9 ± 3.28°) and night (0.8 ± 5.89°), which must be taken into account when conducting acoustic surveys. These results will be useful for calculating the average dorsal aspect target strength (TS) of threeline grunt for accurate estimations of fish abundance.
Highlights
Quantitative echo-sounding has been used effectively to estimate fish abundance since the development of the echo-integration method based on the proportional relationship between fish abundance and acoustic backscattering strength of the animals (e.g., [1])
Video movies recorded with the underwater camera showed that threeline grunt with and without data loggers formed a single school and swam together during the experiment
Since our experimental specimens were caught over Hachirigase sea hill, we believe that the caged fish with and without loggers foraged in a manner similar to wild fish over the sea hill and that the greater variance of body tilt angle at night in the fish cage suggests a foraging behavior of threeline grunt
Summary
Quantitative echo-sounding has been used effectively to estimate fish abundance since the development of the echo-integration method based on the proportional relationship between fish abundance and acoustic backscattering strength of the animals (e.g., [1]). Body tilt angle distributions in swimming fish must be taken into account for accurate estimation of abundance. Optical instruments, such as underwater cameras, have been widely applied to measure the body tilt angles of swimming fish [4,5]. These instruments cannot be used at night because natural light is insufficient and artificial light may change fish behaviors, affecting normal diurnal patterns.
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