Development of turtle releasing system in the pound net: Guiding method of loggerhead sea turtle Caretta caretta with slope of the upper panel

Abstract

To reduce incidental mortality of sea turtles in a pound net, a new releasing system was developed to allow turtles to escape from the bag net. The turtle releasing device with flapped vent has already been developed for small-scaled pound nets, but seems ineffective for large-scaled pound nets because the turtles rarely encounter the device in a large bag net. This study examined the validity of the sloping upper panel for guiding sea turtle to the releasing device in the bag net. Tank experiments were performed with three types of experimental bag nets: a box-shaped net with horizontal upper panel, one with square-pyramid shaped upper panels of 10 degrees (10 degrees slope net), and square-pyramid shaped bag net with upper panel of 20 degrees (20 degrees slope net). Five loggerheads in captivity (SCL: 42.8cm-50.0cm) were prepared. The turtle swam around after entering the bag net, and about five minutes later it started to push its head up against the upper panel. In the 20 degrees slope net, the turtle moved to the shallowest space of the net. This suggested that the sloping upper panel guided the loggerhead turtle to the releasing device in the bag net of a largescaled pound net.