Differences in the behavioral characteristics between green and loggerhead turtles in a setnet bycatch simulation

Sea turtles that have entered the submerged bag net of setnets repeatedly push their heads up against the ceiling netting (referred to here as push-ups) to ascend to breathe. Consecutive push-ups are essential to the success of escaping in a turtle releasing system developed to reduce the incidental death in setnets. Existence of differences in behavioral characteristics were suggested among turtle species, but no detailed evaluation has been performed. The objectives of this study were to establish extraction methods of the consecutive push-ups with a tri-axial acceleration logger, and to clarify the differences in the behavioral characteristics between green Chleonia mydas and loggerhead Caretta caretta turtles under the simulated condition of bycatch in setnets for appropriate adaptation and modification of this system. The behavior of each turtle mounted with an acceleration logger in an experimental bag net was recorded using a video camera. The consecutive push-ups were extracted by filtering the time-series body angle converted from the static surge acceleration and the overall dynamic body acceleration (ODBA) from the triaxial dynamic acceleration. A threshold of 20° in body angle and 1.0 m/s2 in ODBA was detected among 89 % of all consecutive push-ups in green turtles. On the other hand, the ODBA was not adopted as the extract condition in loggerhead turtles, although a threshold of 20° in the body angle detected 93 % correctly. Mode of ODBA (1.0–1.5 m/s2) in the green turtles was larger than that in the loggerhead turtles (0.5–1.0 m/s2) during consecutive push-ups. In contrast, the number and duration of the consecutive push-ups in the green turtles were less than those of loggerhead turtles. Consequently, modifying the gear with a larger inclination angle of the ceiling netting should be considered for the setnets where the bycatch of green turtles dominates.公開予定日: 2023-06-1