Groeneveld et al.: Escapement of Jasus lalandn from traps 



53 



1980; Maynard et al., 1987). These disadvantages are 

 offset by the convenience with which indirect studies 

 measure selectivity under operational conditions. Far 

 fewer direct studies, in which the size distribution of 

 the fished population is known (Millar and Fryer, 1999), 

 have been published, and those that have been pub- 

 lished have included several laboratory studies where 

 the escape of crustaceans from traps was monitored 

 (Krouse and Thomas, 1975; Krouse, 1978; Everson et 

 al., 1992). Direct studies do not recreate true commer- 

 cial conditions, but rather provide a contact-selectivity 

 curve (or retention curve) that quantifies the difference 

 in length distribution between the catch and the popula- 

 tion offish coming in contact with the gear (Millar and 

 Fryer, 1999). This information is useful as a benchmark 

 against which operational, seasonal, and spatial selec- 

 tivity patterns can be measured. 



Commercial fishing for the South African Cape rock 

 lobster (Jasus lalandii) originated in the late nine- 

 teenth century and reached its pinnacle in the 1950s, 

 when nearly 11,000 tons were landed annually (Pol- 

 lock, 1986). However, since then catches have declined 

 markedly, especially during the 1990s, when annual 

 catch restrictions based on the assumption of decreased 

 population strength, reduced the yield to 2000-3000 

 tons per year (Pollock et al., 2000). In response to 

 these operational changes, several recent modifications 

 have been made to the regulations governing gear used 

 in the fishery (Schoeman et al., 2002b). The changes 

 most pertinent to this study took place in 1984, when 

 mesh size was increased from 62 to 100 mm (stretched) 

 to reduce the relative catch of undersize J. lalandii 

 (Schoeman et al., 2002b), and during the early 1990s, 

 when the minimum size limit was reduced from its 

 historic level of 89 mm carapace length (CL) to 75 mm 

 CL (Cockcroft and Payne, 1999; Pollock et al., 2000). 

 Despite these two measures, the proportion of the com- 

 mercial catch <75 mm CL that has to be released re- 

 mains around 35-40% (MCM 1 ). At present, the biomass 

 of the J. lalandii resource that is larger than the mini- 

 mum legal size is estimated at about 6% of its pristine 

 value, whereas the spawning biomass (of mature female 

 rock lobsters) is estimated to be 21% (Johnston, 1998). 

 Consequently, it is clear that the resource is heavily 

 depleted and that there is little scope for wasted produc- 

 tion through unnecessary damage to undersize lobsters 

 (Schoeman et al., 2002a). 



Most studies on trap selectivity of J. lalandii (New- 

 man and Pollock, 1969; Crous, 1976; Pollock and Bey- 

 ers, 1979) predate the changes to mesh and minimum 

 legal size described above and did not provide selectiv- 

 ity curves. In the only recent study, Schoeman et al. 

 (2002a) used the SELECT (Share Each LEngth class's 

 Catch Total) method (Millar, 1992, Milllar and Walsh, 

 1992) to investigate the selectivity properties of vari- 



1 MCM (Marine and Coastal Management). 2002. Unpubl. 

 data. MCM, Martin Hamershclacht St., Cape Town, South 

 Africa. 



ous modifications to commercial and research traps in 

 comparison with the standard 100-mm stretched mesh 

 trap design. This study was indirect, in that it simu- 

 lated commercial fishing and compared catch rates in 

 other traps to those made with a small-mesh (62 mm, 

 stretched) trap, which acted as a control. 



Several processes are involved in the selectivity of 

 traps: namely the attraction of rock lobsters by bait; 

 their ability to enter traps through trap openings of 

 various sizes, shapes, and localities within the trap; 

 their behavior in and around traps; their escapement 

 through the trap opening and their escapement through 

 mesh openings or escape vents (Miller, 1990). The pres- 

 ent study focuses on escapement of captured J. lalandii 

 through the mesh of stretched mesh traps and through 

 trap entrances. The aims are to investigate the relation- 

 ships between CL and other morphometric measures for 

 male rock lobsters in order to use these relationships 

 to estimate theoretical escapement curves for any given 

 mesh size; to compare these curves to observed escape- 

 ment rates through selected meshes in the aquarium; 

 and to extend these comparisons to field conditions. The 

 overall aim is to determine the optimum mesh charac- 

 teristics that maximize efficiency in targeting legal-size 

 male J. lalandii. 



Material and methods 



Mesh size of lobster traps 



Mesh size is defined as the measurement from inside 

 of knot to inside of knot when the net is stretched in 

 the direction of the long diagonal of the meshes, i.e., 

 lengthwise of the net. Netting is made of polyethylene. 

 Commercial rock lobster traps (Fig. 1) are covered with 

 100-mm stretched mesh (or 50-mm bars, also measured 

 from the insides of knots), which are stretched in such 

 a manner over the metal frame that the openings are 

 square. 



Morphometric variables measured 



Following manual trials that involved fitting lobster car- 

 apaces of different sizes through an adjustable square 

 hole, three carapace dimensions were identified as likely 

 to play a role in regulating escapement. These were the 

 following: 1) carapace width (CW), measured laterally, 

 across the widest point of the carapace; 2) carapace 

 depth (CD), measured dorsoventrally, extending from 

 the highest point of the dorsal carapace surface to 

 the lowest point on the ventral surface of the thoracic 

 plate; and 3) carapace base (CB), measured ventrally, 

 between the distal edges of the second segment of the 

 last walking legs, with the legs folded flush against 

 the carapace. 



Each of these dimensions was measured (±1 mm) for 

 each of 169 male rock lobsters caught in research traps 

 deployed off the Cape Peninsula between 1999 and 

 2002. Corresponding data regarding carapace length 



