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

Cod-end drag as a function of catch size and towing speed

“…These values are likely to overestimate the drag of cod-ends attached to trawls where the flow speed at the mouth of the cod-end is likely to be smaller than the towing speed of the trawl. On the other hand, the cod-end attached to a trawl also oscillates in the longitudinal direction in response to the dynamic interaction of the fishing vessel, the fishing gear and the sea state (O'Neill et al, 2003), in which case the drag will be larger than that of the steady state.…”

Performance of an Active Stimulating Device Using a Rope Kite or Array in the Cod End to Reduce Juvenile by-catch

“…The pulsing motion of the cod end affected the escape of juvenile haddock in the North Sea bottom trawl; the period of the cod end was 4-8 s due to trawler motion when wave heights were 1.7-2.8 m (O'Neill et al, 2003). Kim and Jang (2005) estimated the period of the swimming pattern of black porgy in the cod end to be about 3-5 s, and Kim and Wardle (2006) estimated the period for acceleration of the swimming speed of flatfish near a ground rope to range from 2 to 4 s. The rotating period of the rope kite in this experiment was similar to these results and might act to intervene or disturb steady swimming rhythms such as an optomotor response (Wardle, 1993;Kim and Wardle, 2008).…”

“…Studies have demonstrated that the cod end in a bottom trawl has a posterior circular expanding shape that increases in relation to the size of the catch, while the diameter decreases in the anterior of the cod end (Robertson, 1986;O'Neill et al, 2003). Changes in cod end shape appear to be gradual and somewhat catch dependent and to depend on towing time.…”

Effect of catch size and shape on the selectivity of diamond mesh cod-ends

“…It has been observed that this main escape process is often partly passive with fish moving around relative to the cod-end. O'Neill et al (2003) suggest that this motion is caused by the movement of the cod-end itself, and not by turbulence generated by the impenetrable, tightly packed bulb-shaped catch tumbling the fish ahead of it. Many fish reach the open meshes and if their head goes through a mesh, then the fish often has enough energy left to squeeze through, aided by vigorous tail movements and the influence of the water flow outside the cod-end on its head.…”

“…Some of the results are reported in Main and Sangster (1981, 1983, 1991 and in Wardle (1983Wardle ( , 1993. O'Neill et al (2003) have provided information on fish escapement behaviour in dynamic moving cod-ends. The behaviour pattern described below is based on these references and they form the basis for the models of fish behaviour used in this paper.…”