Acoustic characteristics of a potentially important macroalgae, Sargassum horneri, for coastal fisheries

Abstract

A species of macroalgae, Sargassum horneri, which has gas-filled vesicles, plays important roles as nursery grounds, feeding grounds, and hiding grounds for coastal fauna. In addition, it recently emerged as a new coastal fishery resource in Japan. However, its acoustic reflection intensity is unknown, resulting in the impossibility of quantitatively estimating its biomass via an acoustic survey with a quantitative echosounder. We measured the target strength (dB re 1 m2) of S. horneri and clarified its characteristics. The measurements were taken using a 120 kHz quantitative echosounder in the fishing port of Iwate Prefecture in June and July 2014. Seven samples were collected by scuba diving in the coastal waters surrounding the port. Each individual sample was divided every 30 cm, and each divided layer was suspended at a depth of about 4–5 m, and its target strength was measured. Then, we divided S. horneri into vesicles and other parts. Each part was exsiccated at 80 °C for over 10 h. The dry weights of the vesicle and algal body were then measured. The weight of the vesicles of S. horneri tended to increase with an increase in algal body weight. The target strength increased with increasing algal body weight (Target strength = 11.1 log10Weight -71.1, R2 = 0.81) and vesicle weight. We considered that the contribution of the gas in the vesicles to the target strength was high, which is similar to the swim bladder of fish, as the acoustic reflection intensity of the vesicle was as strong as what has been reported for the gas in the swim bladder, and the intensity increased owing to the increase in vesicle weight accompanying the increase in algal body weight. Thus, S. horneri biomass could be estimated by acoustic survey using the target strength revealed in our study.