A newly developed soft-type turtle releasing device (Soft-TRD) for setnet fisheries
2021
Shiode, Daisuke | Shiozawa, Maika | Hu, Fuxiang | Tokai, Tadashi | Hirai, Yoshio
This study investigated a newly developed soft-type turtle releasing device (Soft-TRD) for setnets. Some setnet fish chambers (bag nets) are open at the sea surface, while others are closed (with ceiling netting) and submerged. Sea turtles that stray into submerged bag nets often drown because the ceiling netting prevents them from swimming up to the surface to breathe. The Soft-TRD has a slit that functions as an escape outlet at the center of a 2 m × 2 m netting. The escape slit remains closed, due to the buoyancy of a float attached at each end of the slit, and prevents fish from getting out. Turtles can push the escape slit upward, which opens and allows the turtles to escape. To decide the optimum design of the Soft-TRD, a turtle releasing trial was conducted in an outdoor water tank. Four loggerhead turtles (Caretta caretta) (standard carapace length (SCL): 67.1–72.8 cm) and five green turtles (Chelonia mydas) (SCL: 42.4–63.4 cm) were used for the trial. A single turtle was placed into the experimental bag net where the Soft-TRD (six models in total) was mounted at the center part of the ceiling, and its behavior was observed. All turtles successfully escaped the model with a 150 cm length escape slit, 10 cm overlaying width, and a 4 kgf buoyancy float at each end. The escape slit immediately closed after the turtles escaped. A sea trial (same model) was also performed using a submerged bag net (30 × 10 × 10 m) for two months. Approximately 90% (seven of the eight) of the entrapped loggerhead turtles successfully escaped out through the Soft-TRD. Fishing was not interrupted by the introduction of this device. The use of the Soft-TRD would be helpful in releasing sea turtles from the setnets and contribute to sea turtle conservation efforts.
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