Analysis of turbulence and tilt by in-situ measurements inside the codend of a shrimp beam trawl

The turbulence flow inside the codend and tilt of the codend in a shrimp beam trawl was measured with a Nortek Vector and the towline tension during five fishing operations; data collected for turbulence intensity with codend tilt was analysed under towing speeds. The turbulence intensity at a point in the posterior upper part of the codend ranged from 5 to 40% and decreased significantly by mean flow velocity (0.1–0.9 m s−1) inside the codend. Simultaneously, the shorter periods of the turbulence flow in the towing or perpendicular direction were located between 2 and 8 s by the FFT method and the Morlet wavelet method. Pitch and roll of the codend also varied each other within S.D<±6° and their shorter period was shown mostly as 3 s around. However, turbulence flow velocities inside the codend were not related to tilt changes or towline tension, which increased significantly with towing speed. As a result, turbulence intensity and periodicity in the codend could affect the manoeuvring of fish swimming and the application of active stimulating devices.

► The 3-D flow, tilt and tension were measured inside the codend of a shrimp beam trawl. ► The turbulence intensity ranged 5–40% decreasing significantly by mean flow velocity. ► The periods of flow velocity were related to the period of tidal flow or towline tension. ► The turbulence and tilt in the codend could affect the manoeuvring of fish swimming.