Broadhurst et al.: Effects of square-mesh panels in codends and of haulback delay on bycatch reduction 



415 



eastern blue spot flathead, Platycephalus caeruleo- 

 punctatus, and smooth bug, Ibacus sp. 



The paired comparisons done in experiment 1 were 

 analyzed with one-tailed, paired ('-tests. Data from 

 experiment 2 were analyzed with two-factor analy- 

 sis of variance (Underwood, 1981) after testing for 

 homogeneity of variances (Cochran's test). Further, 

 treating the data from each haulback-delay period 

 separately, we compared data from the pairing of the 

 85-mm-long and control codends, using one-tailed, 

 paired /-tests. Data from the 85-mm-long codend and 

 its control (which used the haulback delay of 10-15 

 s in experiment 2) were combined with data from 

 these codends in experiment 1, to provide a larger 

 dataset for analysis of the results for the 85-mm-long 

 codend. Size frequencies of prawns and red spot whit- 

 ing from both experiments and the combined data 

 were compared by using two-sample Kolmogorov- 

 Smirnov tests (P=0.05). 



Results 



Experiment 1 — evaluation of codends 



Both the 85-mm-long and 85-mm-wide codends sig- 

 nificantly reduced the weight of discarded bycatch 

 (means reduced by 46% and 38%, respectively) (Fig. 

 2A; Table 1). The 85-mm-wide codend significantly 

 reduced the number and weight of discarded red spot 

 whiting (by 71% and 75%), and the number of trash 

 species (Fig. 2, D-F; Table 1). There were insuffi- 

 cient data from the 85-mm-long codend to analyze 

 data for red spot whiting ( <2 fish per replicate). Nei- 

 ther of the codends with square-mesh panels signifi- 

 cantly reduced the catch of prawns (Table 1), al- 

 though the mean catches were 7.5% lower in the 85 

 mm long codend and 2.5% lower in the 85-mm-wide 

 codend (Fig. 2G). Apart from a significant reduction 

 in the number of trash species with the use of the 

 85-mm-long codend ( Fig. 2F; Table 1 ), there were no 

 other detectable differences between the two codends 

 with square-mesh panels. Two-sample Kolmogorov- 

 Smirnov tests comparing the size-frequency distri- 

 butions for the king prawns and red spot whiting 

 measured from each sample showed no significant 

 differences in the relative size compositions between 

 any of the codends tested. 



Experiment 2 — effects of haulback delay 



There were no significant effects due to haulback 

 delay on the differences in catch between the control 

 and 85-mm-long codends for the weight of bycatch 

 and prawns (Fig. 3, A-B; Table 2). However, haulback 



delay significantly increased the difference in catch 

 between the control and 85-mm-long codends for the 

 number and weight of red spot whiting and decreased 

 the weight of retained cuttlefish (Fig. 3, C-E; Table 

 2). There were no significant effects among any of 

 the other variables (i.e. retained octopus, discarded 

 eastern blue-spot flathead, smooth bug, and trash 

 species) due to haulback delay. The number and 

 weight of red spot whiting were significantly differ- 



