Sinking performance of new biplane depressor for constant-depth ichthyoplankton sampler at horizonal sampling

A constant-depth ichthyoplankton sampler is vital equipment for monitoring fish recruitment and assessing fishery stocks. Its depressor, a critical component, maintains towing stability during sampling by balancing the sinking force of the depressor and the drag force of the sampler. Conventional depressors often fail to maintain this equilibrium due to their limited sinking force. This study proposes a biplane depressor, inspired by the aerodynamics of biplane hyper-lift trawl doors. In the hydrodynamic experiment, the maximum sinking force coefficient for the proposed biplane depressor with a stagger angle of 30° reached 2.02, which was significantly higher than that for conventional depressors (less than 1.60). Moreover, substituting metal with canvas for 40% of the wing area reduced the weight without compromising sinking performance (maximum sinking force coefficient: 1.86). A towing experiment demonstrated a consistent sinking force and towing stability for the depressor with and without canvas. Operating angles of attack of 27° and 23° are recommended for the depressor made of all metal and a combination of metal and canvas, respectively. The results indicate the potential of the proposed biplane depressor to replace conventional depressors to enhance the constant-depth performance of the sampler and ensure reliable sample collection for ichthyoplankton surveys. • A new biplane depressor was designed to improve the poor sinking performance of conventional depressors in constant-depth ichthyoplankton samplers (CDIS). • Hydrodynamic experiments confirmed that the maximum sinking force coefficient for the proposed biplane depressor with a stagger angle of 30°was 2.02, which was significantly higher than that of conventional depressors (less than 1.60). • Even if 40% of the wing area of the proposed biplane depressor was replaced by canvas, the maximum sinking force coefficient remained at 1.86. • Good towing stability of the proposed biplane depressor with and without canvas was verified through towing experiments.