An investigation of the relationship between sea state induced vessel motion and cod-end selection

Abstract

The relationship between sea state induced vessel motion and cod-end selection is investigated. The paper is divided into three parts. Part 1 reports on sea trials to investigate the relationship between vessel motion and cod-end dynamics. By comparing the average period of longitudinal cod-end pulsing to the most important cyclic component of the tension in the trawl warps and the most important cyclic component of the linear accelerations of the fishing vessel, it is established that the pulsing of the cod-end is a response to sea state induced vessel motion. Part 2 looks at the hydrodynamics and catch dynamics of a cod-end pulsing in a flume tank. Experiments are carried out on a half scale cod-end where the towing warp is attached to the rotating arm of a hydraulic motor and the catch is simulated using water filled balloons. Three categories of longitudinal cod-end motion are identified and a qualitative description of the hydrodynamic forces that are acting is presented. A number of observations are made concerning the catch dynamics and it is shown that, from a fixed reference frame, the displacement of the balloons is essentially linear and has no rotational component. It is concluded that the dynamic forces acting on the catch must generally be small. Part 3 analyses direct observations of the hydrodynamics, catch dynamics and fish behaviour of pulsing cod-ends at sea. By comparing these observations with the results of the flume tank trials it is concluded that longitudinal pulsing of the cod-end is the predominant factor generating dynamic movement of the cod-end; that the dynamic forces acting on the fish ahead of the catch are small; and that the main component of the movement of these fish relative to the cod-end arises as a result of the displacement of the cod-end. In the light of these results, an analysis of recorded observations of fish in pulsing cod-ends provides insight into some of the associated hydrodynamic and behavioural mechanisms which may affect the active and passive selection of fish in the cod-end. In particular, a cyclic tilting behaviour and reductions in flow speed are used to explain the dependence that is found between fish escape ahead of the catch and the cod-end motion. When allied to the result of part 1 of this study a direct link is established between sea state induced vessel motion and cod-end selection.