52 



Abstract — Metal-framed traps cov- 

 ered with polyethylene mesh used 

 in the fishery for the South African 

 Cape rock lobster (Jasus lalandii) 

 incidentally capture large numbers 

 of undersize (<75 mm CD specimens. 

 Air-exposure, handling, and release 

 procedures affect captured rock lob- 

 sters and reduce the productivity of 

 the stock, which is heavily fished. 

 Optimally, traps should retain legal- 

 size rock lobsters and allow sublegal 

 animals to escape before traps are 

 hauled. Escapement, based on lobster 

 morphometric measurements, through 

 meshes of 62 mm, 75 mm, and 100 

 mm was investigated theoretically 

 under controlled conditions in an 

 aquarium, and during field trials. 

 SELECT models were used to model 

 escapement, wherever appropriate. 

 Size-selectivity curves based on the 

 logistic model fitted the aquarium and 

 field data better than asymmetrical 

 Richards curves. The lobster length 

 at 50% retention (L 50 ) on the escape- 

 ment curve for 100-mm mesh in the 

 aquarium (75.5 mm CL) approximated 

 the minimum legal size (75 mm CL); 

 however estimates of Z/ 50 increased to 

 77.4 mm in field trials where trap- 

 entrances were sealed, and to 82.2 

 mm where trap-entrances were open. 

 Therfore, rock lobsters that cannot 

 escape through the mesh of sealed 

 field traps do so through the trap 

 entrance of open traps. By contrast, 

 the wider selection range and lower 

 L 25 of field, compared to aquarium, 

 trials ^SR = 8.2 mm vs. 2.6 mm; 

 L., 5 =73.4 mm vs. 74.1 mm), indicate 

 that small lobsters that should be 

 able to escape from 100-mm mesh 

 traps do not always do so. Escape- 

 ment from 62-mm mesh traps with 

 open entrance funnels increased by 

 40-60% over sealed traps. The find- 

 ings of this study with a known size 

 distribution, are related to those of a 

 recent indirect (comparative) study for 

 the same species, and implications for 

 trap surveys, commercial catch rates, 

 and ghost fishing are discussed. 



Escapement of the Cape rock lobster 

 (Jasus lalandii) through the mesh 

 and entrance of commercial traps 



Johan C. Groeneveld 



Marine and Coastal Management 



5 lh floor Foretrust Building 



Martin Hamerschlacht Street, Foreshore 



Cape Town, South Africa 



E-mail address. Jgroenev<a>deat.gov.za 



Jimmy P. Khanyile 



National Research Foundation 



P.O. Box 2600 



Pretoria 0001, South Africa 



David S. Schoeman 



Department of Zoology 

 University of Port Elizabeth 

 Port Elizabeth 6031, South Africa 



Manuscript submitted 20 March 2003 

 to the Scientific Editor. 



Manuscript approved for publication 

 1 July 2003 by the Scientific Editor. 



Fish Bull. 103:52-62 (2005). 



The traps used in lobster and crab 

 fisheries are a versatile fishing gear 

 that can be modified to target specific 

 species and size ranges through choice 

 of design and bait (Miller, 1990). Selec- 

 tion by traps of only the desired size 

 classes reduces sorting time and may 

 increase the catch rates of legal-size 

 animals (Fogarty and Borden, 1980; 

 Everson et al., 1992; Rosa-Pacheco 

 and Ramirez-Rodriguez, 1996). Cap- 

 ture, sorting and release procedures 

 have furthermore been implicated in 

 accidental and stress-induced mor- 

 talities (Brown and Caputi, 1983; 

 1985; Hunt et al., 1986), as well as 

 in sublethal injuries, such as limb 

 loss (legs or antennae), which may 

 retard somatic growth (Davis, 1981; 

 Brown and Caputi, 1985). Air expo- 

 sure, even over short periods, can 

 induce behavioral changes such as 

 reduced responsiveness to threatening 

 stimuli (Vermeer, 1987) and lead to 

 higher predation risk among released 

 animals (Brown and Caputi, 1983). 

 Furthermore, displacement from 

 home reefs disrupts feeding behav- 

 ior and can affect growth increments 

 (Brown and Caputi, 1985). Manag- 

 ers of many crustacean trap fisher- 

 ies have responded to these problems 

 by introducing escape vents of vari- 

 ous sizes and shapes (Krouse, 1989; 



Miller, 1990; Everson et al., 1992; 

 Arana and Ziller, 1994; Rosa-Pacheco 

 and Ramirez-Rodriguez, 1996; Treble 

 et al., 1998; Schoeman et al., 2002a), 

 because they successfully allow under- 

 size specimens to escape (Arana and 

 Ziller, 1994; Treble et al. 1998). 



In fisheries management, size selec- 

 tivity curves are important for esti- 

 mates of incidental mortality, recruit- 

 ment in yield-per-recruit analysis, 

 and age- and length-based popula- 

 tion models (Millar and Fryer, 1999). 

 Notably, size selectivity can be used 

 to evaluate the minimum legal size 

 (MLS) and the effects of changing 

 escape vent or mesh size regulations 

 on the future productivity of the re- 

 source (Treble et al., 1998). 



Most selectivity studies on which 

 mesh- or escape vent size are based 

 are comparative (indirect), imply- 

 ing that the size distribution of the 

 population is unknown and that 

 variants of the same gear type are 

 fished simultaneously (Millar and 

 Fryer, 1999). Results from indirect 

 studies can, however, be influenced 

 by trap soak times, trap saturation 

 effects (Miller, 1990), seasonal size 

 and sex-specific patterns in catchabil- 

 ity (Pollock and Beyers, 1979), and by 

 differences in morphometric ratios of 

 subpopulations (Fogarty and Borden, 



