Behavioural characteristics of loggerhead turtles (Caretta caretta) in a submerged bag net of a setnet observed in a bycatch simulation and the development of a turtle releasing device

Abstract

A turtle releasing device (TRD) can reduce the mortality of bycatch in setnets. Turtles that have entered the submerged bag net of setnets repeatedly push their heads up against the ceiling net (referred to here as a push‐up) in an effort to ascend to breathe. To aid the successful escape of turtles from the setnet, it is essential to understand the turtle's push‐up behaviour, as they need to reach the TRD and open its flap door. The objectives of this study were to clarify when turtles start to perform push‐ups, how these push‐ups change, and what factors affect push‐ups under the simulated condition of bycatch. A depth, temperature, and tri‐axial acceleration data logger and a video camera were employed to determine vertical movement, ambient temperature, the level of activity, and flipper beat frequency (FBF) of the turtles. Ten trials of 10–30 minutes for five subadult loggerhead turtles (Caretta caretta) were conducted using a submerged bag net (30 × 10 × 10 m) of a setnet in Mie Prefecture, Japan. Turtles started to perform push‐ups 5.1 ± 3.3 minutes after the start of each trial. FBF increased as more time elapsed in 35% of ascent phases recorded, and the level of activity during the following push‐ups was significantly higher than during other push‐ups. The level of activity in each sequence of push‐ups increased as more time elapsed; however, it started to decrease from 9.8 minutes after the first push‐up at the earliest. Therefore, the TRD should be designed to enable turtles to escape within approximately 10 minutes. The number of push‐ups in a single sequence, the activity level, and FBF were affected by water temperature. Therefore, the closing force of the TRD flap door needs to be changed according to the different seasons and regions.

Water-Tank and Semi-Field Experimental Investigation of a Hose Net Fish-Hauling Device for Next-Generation Set-Net Fishing

“…Several studies have been conducted, including research on optimizing mesh sizes for set-net [17], analyzing the behavioral characteristics of fish within these nets and estimating fish species [18,19], and the development of real-time monitoring and estimation systems tailored for set net fishing [20]. Notably, research is underway to understand the biological behavior of sea turtles in set-net [21]. However, despite these advancements, the traditional fish-hauling method from the 1950s is still in use.…”