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Fishery Bulletin 103(1) 



by Pythagoras's theorem (r=4.470; df =39; P«0.001). 

 In addition, repairs to torn meshes often leave openings 

 that are somewhat larger than 100 mm and that are not 

 square (Groeneveld, personal, observ. ). The wider SR of 

 the selectivity curve for field data compared to the tight 

 SR of the aquarium curves supports this "unevenly 

 stretched mesh" hypothesis. 



Paradoxically, a 70-mm-CL lobster, which has a 1% 

 chance of being retained by a 100 mm mesh in the 

 laboratory has an 11% probability of being retained by 

 a trap with the same mesh in the field (even when its 

 entrances are sealed). Thus, some rock lobsters that 

 should, and could have escaped through the 100 mm 

 mesh of the field traps did not. Schoeman et al. (2002a) 

 suggested that small rock lobsters that can escape do 

 not always do so because they use the trap as a refuge 

 against predators. Alternatively, overnight soak times 

 (as used in the field trials) may be too short for all 

 the small rock lobsters to escape. The probability of 



Table 2 



Statistics from SELECT analysis for the field escapement 

 trials. Values in parentheses are asymptotic standard 

 errors sensu Millar (1993). These standard errors are 

 provided only for the best model fits for each of the vari- 

 ous categories of data. Note that all hypothesis tests were 

 conducted by using deviance residuals for the full model 

 and their degrees of freedom (see text for explanation). 



H : data have binomial distribution 

 (i.e., data are not overdispersed) 



escape is much reduced during hauling because captive 

 specimens are then pressed into a tight mass within the 

 fine-mesh (62-mm) codend of the trap. 



No escapement from sealed 62-mm mesh traps was 

 expected during field trials, based on the aquarium 

 L 50 of 46.4 (±0.3) mm and the size range of lobsters 

 used in the field (60 mm-95 mm CL). Nevertheless, 

 small losses (0-18%, depending on lobster size; see 

 Fig. 5) did occur. Only two explanations are possi- 

 ble, namely: 1) that lobsters still managed to escaped 

 through the mesh of the sealed 62-mm traps, despite 

 precautions taken to ensure that the meshes of these 

 traps were undamaged and that trap openings were 

 properly sealed; and 2) that some individuals sus- 

 tained injuries during exposure and handling, and 

 subsequently were cannibalized by the healthy rock 

 lobsters in the traps. This second conclusion is sup- 

 ported by the presence of shell fragments observed 

 in traps after their retrieval. Because these regres- 

 sions had the same positive slopes, it seems likely 

 that smaller rock lobsters would be more susceptible 

 to injury and cannibalism than larger animals, and 

 their susceptibility holds irrespective of whether the 

 trap entrance is sealed or not. 



Escapement from 62-mm mesh traps with open en- 

 trance funnels increased by 40-60% compared to es- 

 capement from traps with sealed traps (Fig. 5). This 

 finding has significant implications for the FIMS, which 

 relies on catches made by 62-mm mesh traps and is con- 

 ducted annually as a measure of the relative abundance 

 of the J. lalandii resource. During a survey, it is as- 

 sumed that all the Cape rock lobsters that are captured 

 are retained and that trap-selection is uniform across 

 all the size classes of these lobsters (Johnston, 1998). 

 It appears that neither of these two assumptions can 

 be met: significant escapement does occur through the 

 trap entrance and there is a greater retention of larger 

 specimens than smaller specimens . 



When the trap entrance was left open in the 100- 

 mm mesh field trials, L 50 increased to 82.3 mm (from 

 77.4 mm in sealed traps), thus indicating that captive 

 Cape rock lobsters can and do use the trap entrance of 

 commercial traps to escape. The open traps also have 

 a wider SR of 10.1 mm (compared to 8.2 mm in sealed 

 traps), and therefore animals with a CL of >87 mm 

 (L 75 =87.3 mm), which are very unlikely to be able to 

 get through the mesh apertures, will still be able to use 

 the trap entrance to exit. The presence of this escape 

 vent implies that there is little danger of ghost-fishing 

 when using this trap type and that Cape rock lobsters 

 of all sizes should be able to vacate the trap once the 

 bait has been consumed. From a commercial viewpoint, 

 however, the problem of leaving traps in the water for 

 too long is that legal-size specimens, which cannot fit 

 through the mesh, will escape through the entrance, 

 thus decreasing catch rates considerably. 



The aquarium result (L 50 =75.1 mm) is considered 

 the most accurate direct measurement of the selectiv- 

 ity of 100-mm square mesh for J. lalandii, because 

 care was taken to ensure that the mesh was stretched 



