Behavior of sea turtles to a turtle releasing device (TRD) for set nets of the mid-water and sea-bottom bagnet type

Abstract

A turtle releasing device (TRD) was developed to reduce incidental mortality of sea turtles in set nets with a bag net at medium depth or the bottom. The TRD consisted of a 1.0-meter-square escape vent on a slightly deformable plastic net of 1.5 m square and a flap door covering the vent with an automatic door closing system. The frame of the flap door was made of stainless steel bars, and the part on the fulcrum side was fixed on the plastic net. The end of the flap door deformed the plastic net when a sea turtle opened the door, and the elastic restoring force of the plastic net closed the door automatically. A turtle releasing experiment was conducted in an outdoor water tank, using an experimental bag net of which the TRD was mounted at the center part of the ceiling. Fifteen loggerhead turtles were prepared for the experiment. A single turtle was put into the experimental bag net, and its escaping behavior was observed. All of the loggerhead turtles successfully escaped from the experimental net through the TRD. The flap door closed automatically after the turtles escaped. The TRD is helpful to release sea turtles from passive fishing gear such as set nets.