Horizontal and vertical movement patterns of sunfish off eastern Taiwan

Abstract

The horizontal and vertical movements of two large sunfishes (TL > 200 cm) were investigated using pop-up satellite archival tags off eastern Taiwan in 2017–2018. One tag attached to Mola mola popped-up in the South China Sea after 148 days at-liberty and another on M. alexandrini was recovered after 18 days. The most probable track, estimated by the Kalman filter with matching sea surface temperature, suggested that M. mola moved in a northerly direction to the East China Sea, went southward back to Taiwan with the tag detaching near the coast of Miyakojima and Okinawa Island, Japan. During these movements, the fish experienced thermal fronts and moved against the prevailing current. Both fish exhibited distinct diel oscillations in their vertical movements. During daytime periods, tagged fish spent most of their time (>72%) below the thermocline and occasionally ascended to the surface and experienced water temperatures of 12–16 °C. During nighttime periods, vertical movements were confined to the mixed layer with water temperatures ranging from 18 to 24 °C. Depth patterns were different between December and May when descending dives were correlated with strength of the thermocline. These movements suggested that sunfish shifted their horizontal and vertical patterns in response to changing ambient water temperature, thermocline structure, and prey availability.