Estimation of the Propulsive Force Generated by Loggerhead Turtles Caretta Caretta for Appropriate Design of Turtle Releasing Device in Set Nets

Abstract

Turtle releasing device （TRD） has been developed to reduce incidental deaths of sea turtles in a submerged bag net of a set net. The TRD consists of a 1m square escape vent attached to the roof net of the bag net and a flap door covering the vent. When turtles need to breathe, they start pushing their heads up continuously （defined as pushing up）. If they open the flap door by pushing up, they can spontaneously escape out through the TRD. Hence, to confirm the design adequacy of the TRD, both the closing force of the flap door and the propulsive force of turtles should be confirmed. In this study, the regression equation of peak force value（ N）, defined as the peak value of the propulsive force generated by each flipper beat, with flipper beat frequency （Hz） and standard carapace length （m） was obtained from the results of tank experiments. And, peak force values of loggerhead turtles pushing up in the submerged bag net were estimated from the equation. Significant regression equation was obtained as follow: lnFP = 1.81lnf + 1.91lnl + 6.26, where FP, f, and l indicate the peak force value, flipper beat frequency, and standard carapace length, respectively. Estimated values by the equation corresponded well with the measurement values. More than 80% of estimated peak force values during pushing up were larger than 34N that is the force where the flap door of the TRD can close in a current of 0.8 knots from the fulcrum side. The design of the TRD is appropriate for turtles to escape out.