Juvenile swimming performance of three fish species on an exposed sandy beach in Japan

Surf zones of sandy beaches, physically dynamic environments due to high wave exposure, provide an important nursery habitat for the juveniles of many marine fish species. This study was specifically aimed at clarifying the juvenile swimming and positioning abilities of three surf zone species (Mugil cephalus cephalus, Terapon jarbua and Trachinotus baillonii), using an experimental tank. Average critical swimming speed was greater for M. cephalus cephalus (47.03 cm s− 1) than in the other two species (T. jarbua = 22.79 cm s− 1, T. baillonii = 26.16 cm s− 1) and critical swimming speed standardized by total length tended to be greater for M. cephalus cephalus than in the other two species. The greater swimming ability of M. cephalus cephalus was primarily due to greater propulsive ratio, muscle ratio and body fineness ratio. The positioning ability, defined as the amplitude of fish oscillations relative to water flow, did not differ significantly among the three species. The fish amplitude was about 0.15–0.36 times that of water amplitude. The lack of significant differences in positioning ability, despite the greater swimming ability of M. cephalus cephalus, was due to greater fineness ratios and shorter nose to pectoral fin base distances, resulting in greater drag when the body was turned. The results of present study suggested that juvenile surf zone fishes do not necessarily have significantly greater swimming ability in order to resist the effects of high wave exposure.