Broadhurst et al : Flow-related effects on prawn-trawl codends 



position nos. 1, 3, and 4 (Fig. 3). Compared with the 

 100 panel codend, there was significantly less water 

 flow in the 200 panel codend with a catch of 70 kg at 

 position no. 1 (0.115 m/s difference between means) 

 and at position no. 3 with 50 kg (difference of 0.102 

 m/s) (Fig. 3, A and C; Table 2). Conversely, at posi- 

 tion no. 4, SNK tests detected an increase in the mean 

 water flow in the 200 panel codend with a catch of 

 30 kg, compared with the 100 panel codend (differ- 

 ence of 0.067 m/s) (Fig. 3D). There was also a simi- 

 lar, although not significant result for 50 kg at this 

 position. There were no other significant differences, 

 although, at position nos. 2 and 3, there were simi- 

 lar trends across all weights (i.e. a reduction in wa- 

 ter flow in the 200 panel codend) (Fig. 3, B and C). 



Discussion 



This study showed that the weight of catches and 

 the configuration of the posterior section in codends 

 can have significant effects on the displacement of 

 water in the anterior section of codends. In inter- 

 preting these results, it is important to note that 

 under the simulated conditions in the present study, 

 water was forced through a stationary trawl (at 1.2 

 m/s). Any localized displacements of water forwards 

 in the codends examined in this study, therefore, can 

 be expressed as a reduction in the flow entering the 

 trawl and calculated by subtraction from 1.2 m/s. 



The flow of water at all positions in the four 

 codends tested was less than 1.2 m/s, indicating that 

 there was displacement of water anterior to the catch. 

 However, the degree of this anterior water displace- 

 ment varied significantly between the codends tested 

 in each experiment (Figs. 2 and 3). For example, in 

 experiment 1, the 200 commercial codend showed a 

 significant increase (compared to the 100 commer- 

 cial codend) in the displacement of water forwards 

 at position no. 3 across all weights of catch (differ- 

 ence in mean flow of up to 0.203 m/s [Fig. 2; Table 

 1]). These differences in flow may be attributed pri- 



marily to the distribution of the balloons used to 

 simulate catch in the two codends and consequent 

 changes in codend geometry. In the 200 commercial 

 codend, the balloons were observed to spread out 

 evenly in the posterior section, providing a greater 

 surface area of catch incidental to the flow than in 

 the 100 commercial codend. This effect, combined 

 with the increase in the area of twine in the 200 com- 

 mercial codend probably caused an increase in the 

 displacement of water forwards in this codend. 



The above effects in the 200 commercial codend 

 were also detected at position no. 2 with 30 kg of 

 balloons and, although ANOVA failed to detect sig- 

 nificant differences for the other weights at this po- 

 sition, the trends in the results were similar (i.e. a 

 reduction in flow in the 200 commercial codend com- 

 pared with the 100 commercial codend [Fig. 2; Table 

 1] >. At position no. 1, for all weights, the force of the 

 displaced water in front of the 200 commercial codend 

 had dissipated to the extent where there were no sig- 

 nificant differences between the two codends (Fig. 2; 

 Table 1). It can be assumed, therefore, that the ma- 

 jor influence of increased codend circumference on 

 water displacement in the middle of the 200 commer- 

 cial codend probably occurred up to some point between 

 position nos. 2 and 3 (560 mm to 1120 mm from the 

 end of the codend), in relation to the weight of catch. 



In experiment 2, the effects of increased codend 

 circumference on water displacement under the 

 square-mesh panel were detectable at a greater dis- 

 tance from the end of the codend than those described 

 above. Compared with the 100 panel codend, there 

 were significant reductions in flow (corresponding to 

 an increased displacement of water forwards) in the 

 200 panel codend at position nos. 1 and 3 (2200 mm 

 and 1490 mm from end of the codend) with a weight 

 of catch of 70 kg and 50 kg, respectively (mean dif- 

 ferences in flow of 0.115 m/s and 0.102 m/s, respec- 

 tively) (Fig. 3, A and C; Table 2). Although not sig- 

 nificant, there were similar trends at position no. 2 

 for each weight and at position no. 3 for 30 kg and 70 

 kg (Fig. 3, B and C; Table 2). In contrast, there was a 



