54 



Fishery Bulletin 103(1) 



Figure 1 



(A) Standard metal-framed traps (0.8 m x 0.5 mxl.35 m high) covered with 

 stretched polyethylene mesh used in the commercial fishery for J. lalandii 

 and during field experiments (note the 100-mm mesh size covering on the com- 

 mercial traps, and the 62-mm mesh size on the codend and entrance funnel). 



(B) Metal-framed escapement cages covered with 62-mm (shown), 75-mm, and 

 100-mm mesh used in the aquarium experiments (frames were 0.6 mx0.6 m, 

 with a depth of 0.25 m). 



(CL), measured mid-dorsally from the posterior edge 

 of the carapace to the anterior tip of the rostral spine, 

 were also collected. This was done because CL is the 

 dimension most frequently mentioned in legislation 

 pertaining to this species (Schoeman et al., 2002b) and 

 has therefore been the focus of most size-based studies 

 (Newman and Pollock, 1969; Pollock and Beyers, 1979; 

 Schoeman et al., 2002a). Relationships between the CL 

 and each of CW, CD, and CB were explored by using 

 simple least-squares regression analyses. 



Theoretical calculations of escapement 



In order to investigate morphological characteristics that 

 physically limit escapement through meshes of various 

 dimensions as a function of CL, digital photographs were 

 taken of the posterior cross section of 46 male carapaces 

 (tail removed) covering a range of sizes between 40 mm 

 CL and 106 mm CL. Using standard graphics software, 

 we superimposed a square on each image to represent 

 a square of polyethylene mesh, similar to that used in 

 a South African rock lobster trap. 



This simulated mesh was orientated so that its base 

 was parallel with the carapace base of the lobster under 

 consideration. It was then proportioned so that each of 

 its sides was equal in length to the corresponding CB. 



Once this procedure had been completed, the simu- 

 lated mesh square was rotated and resized so that we 

 could determine the dimensions of the smallest square 

 through which each lobster could pass. This measure 

 was designated the "critical mesh size" for that image. 

 Critical mesh size was related to CL by using simple 

 linear regression analysis. In this way, the theoretically 

 appropriate mesh aperture required to target all lobster 

 larger than a given size could be predicted from the 

 minimum CL of the target group (for convenience, this 

 CL will be designated the "critical CL"). 



Aquarium trials 



Having addressed the matter of whether or not lobsters 

 theoretically should be able to escape a mesh of given 

 dimensions, the next question to be posed is whether or 

 not they can do so under ideal (laboratory) conditions? 

 For these purposes, three stretched mesh sizes were 

 considered: 1) 62 mm, which coincides with the mesh 

 size used in the commercial fishery prior to 1984 and 

 also with the mesh currently used on traps deployed 

 in the Fishery Independent Monitoring Survey (FIMS) 

 (Schoeman et al., 2002a); 2) 100 mm, which corresponds 

 with the mesh currently used on commercial traps for 

 J. lalandii; and 3) 75 mm, which was used to provide 



