Groeneveld et al.: Escapement of Jasus lalandii from traps 



59 



Discussion 



This study focuses on escapement of Cape rock lobster 

 (J. lalandii) through mesh openings, and on escape- 

 ment through the trap entrance of commercial traps. 

 Three questions were initially posed, namely: through 

 what mesh size, in theory, can a lobster of given CL 

 escape; are lobsters physically able to escape through 

 this theoretical mesh size, or are there other factors such 

 as orientation and mobility of lobster appendages that 

 prevent escapement; and what proportion of sublegal 

 and legal size lobsters escape through the mesh and 

 trap entrance of commercial traps? In brief, the results 

 showed a weak leak between theoretical values and the 

 ability of the lobsters to escape. 



Carapace base (CB) was isolated as the dimension 

 most likely to limit escapement through stretched 

 square meshes. This dimension superceded carapace 

 width and depth, which have been more widely assumed 

 to be the limiting factors to escapement of lobsters 

 (Treble et al., 1998), mainly because our measurement 

 included the width of the last pair of walking legs, 

 folded flush against the carapace. Experimenting with 

 lobster carapaces and an adjustable square hole showed 

 that the joints of these appendages protrude ventrolater- 

 al^ from the carapace, and the orientation and limited 

 mobility of these appendages would prevent the lobster 

 from escaping. Nevertheless, our theoretical escapement 

 model included all three measurements in the underly- 

 ing computer simulations to determine the appropriate 



mesh aperture required to target all lobsters larger 

 than a given size. 



The theoretical escapement model produced surpris- 

 ingly small values of "critical CL" for all three mesh 

 sizes in comparison with the corresponding selectivity 

 curves from the aquarium experiment. This result im- 

 plies that many rock lobsters that should theoretically 

 not have been able to escape, did so in the aquarium 

 trials. We therefore concluded that the theoretical model 

 was weak and that the mechanics of escapement ap- 

 pear to be more complex than can be shown by simple 

 measurements of the carapace dimensions and may rely 

 also on the orientation of lobsters during escapement 

 (Stasko, 1975). 



Selectivity curves developed from aquarium data in- 

 dicated that an 85-mm-CL lobster should not have been 

 able to escape a 100-mm mesh trap. However, field data 

 indicated that escapement from 100-mm mesh traps 

 with sealed trap openings exceeded 10%. Thus, rock 

 lobsters that should not have been able to escape, ac- 

 cording to aquarium experiments, did escape under 

 field conditions. This result was expected, because the 

 mesh of traps used in the commercial fishery (and field 

 experiments) is often unevenly stretched across the met- 

 al trap-frame, and therefore some openings lose their 

 square dimensions. This unevenness in the stretch of 

 the mesh was clearly illustrated by a random sample of 

 40 knot-to-knot aperture measurements from four 100- 

 mm mesh commercial traps, which had diagonal dimen- 

 sions significantly larger than the 70.71 mm predicted 



