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

Abstract

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 (0.1–0.9ms−1) inside the codend. Simultaneously, the shorter periods of the turbulence flow in the towing or perpendicular direction were located between 2 and 8s 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 3s 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.