Direct measurement of the swimming speed, tailbeat, and body angle of Japanese flounder (Paralichthys olivaceus)

It is well known that flatfish species such as plaice can utilize the selective tidal stream to conduct vertical movements. However, detailed description of actual swimming behaviour is lacking, principally as a result of the difficulties encountered in monitoring the behaviour of flatfish in the open sea. The present study describes the use of a newly developed data-logger in obtaining simultaneous recordings of the swimming speed, depth, tailbeat, and body angle of free-ranging Japanese flounder (Paralichthys olivaceus) in the open sea. Our data indicate that Japanese flounders adopt a tailbeat-and-glide behaviour. They are found to glide downward without tailbeats for propulsion, and only during the ascent phase are tailbeats conducted. Flounders move horizontally at speeds of 0.59–1.23 km d−1 and at a maximum speed of 0.70–0.82 km h−1 in the open sea. Modal flounder swimming speeds are 30–40 cm s−1 (0.57–0.76 and 0.58–0.77 BL s−1), i.e. sometimes lower than the threshold of the speed sensor. In most cases, however, tailbeat oscillations occur at frequencies of 1.2–1.4 Hz. Moreover, flounders travel at a significantly steeper angle during the ascent phase than during the descent phase. In both cases it is believed that flounder optimize the energetic costs of migration, as has been shown for tuna, sharks, and seals.