Groeneveld et al.: Escapement of Jasus lalandu from traps 



55 



information on selectivity for meshes of intermediate 

 aperture dimensions. 



Each of these experimental meshes was used to con- 

 struct an escapement cage by stretching the mesh over 

 a mild-steel frame in order to present square escape 

 apertures of varying dimensions, as determined by 

 the size of the mesh used (Fig. 1). These cages were 

 deployed in an aquarium tank measuring 1.8 mxl.8 

 m and having a depth of 1.5 m. Fresh sea water was 

 continuously supplied to this tank by a through-flow 

 system that regulating water temperature between 12° 

 and \&°C, well within the natural temperature range of 

 J. lalandii (Heydorn, 1969). 



For each mesh size, male rock lobsters of various 

 carapace lengths (373 lobsters measuring 34-91 mm CL 

 for 62-mm mesh; 351 lobsters measuring 34-75 mm CL 

 for 75-mm mesh; and 142 lobsters measuring 70-91 mm 

 CL for 100-mm mesh) were collected live from the sea 

 and transported to the experimental aquarium tank. 

 Care was taken to ensure that approximately equal 

 numbers of lobsters were available for each 2-mm size- 

 class within the respective size ranges, although fewer 

 lobsters tended to be available in size classes towards 

 the ends of the frequency distributions. 



Once at the aquarium, lobsters were placed inside 

 the experimental cages in groups of up to 20 and left 

 for 30 minutes. Individuals that did not escape during 

 this period were gently pushed towards the mesh open- 

 ings, encouraging escapement, where this was possible. 

 Subsequently, the CL frequency distributions were de- 

 termined both for those lobsters that escaped the mesh 

 as well as those that were retained. Several replicate 

 escapement experiments were conducted for each mesh 

 size, but because the experimental cages were too small 

 to hold large numbers of lobsters, replicate selection 

 curves could not be computed. Instead, all data were 

 pooled for each mesh size for further analyses. 



Field trials 



The final question to be posed is whether or not lobsters 

 do escape from traps when afforded the opportunity to 

 do so under field conditions? To address this problem, 

 field trials were undertaken off the Western Cape Pen- 

 insula during monthly sampling sessions conducted by 

 the research vessel Sardinops in July 2000 and from 

 December 2001 to March 2002— a total of five distinct 

 sampling surveys. 



Four categories of standard rock lobster traps (Fig. 

 1) were employed: 1) 62-mm stretched mesh, with en- 

 trance funnels open; 2) 62-mm stretched mesh, with en- 

 trance funnels blocked by a fine-mesh insert; 3) 100-mm 

 stretched mesh, with entrance funnels open; and 4) 100- 

 mm stretched mesh, with entrance funnels blocked. 



Duplicate bottom long-lines consisting of 10 traps 

 each were prepared, of which six were normal commer- 

 cial traps, and the remaining four were experimental 

 traps, and these 10 traps were spread in haphazard 

 order along the line, excluding the end traps. Into each 

 trap was placed a sample of approximately 40 male rock 



lobsters, each of which had been measured (CL) and 

 marked by cutting a notch in its uropod. In this way, 

 it was possible to distinguish between lobsters that had 

 been placed in the trap and those that had entered the 

 trap of their own accord. 



Experimental traps were deployed without bait, in 

 order to limit their ability to attract lobsters and also 

 to remove one of the prime incentives that captive lob- 

 sters might have to remain in a trap, even when it 

 could escape. These trap lines were soaked overnight 

 and on their retrieval, each remaining lobster was re- 

 measured (CL) and inspected to identify specimens that 

 had entered the traps voluntarily. Eight replicates were 

 completed for each of the four categories of traps. 



Construction of selectivity curves 



The contact-selection curves (sensu Millar and Fryer, 

 1999) for the meshes used in the laboratory and field 

 trials were modeled by using the SELECT method 

 (Millar and Walsh, 1992) as applied to covered codend 

 experiments (Millar and Fryer, 1999). We felt that this 

 approach was warranted because we collected data with 

 respect to lobsters in both a "codend" (those retained 

 in the traps) and a "cover" (those that escaped, but for 

 which data were available by inference). 



The logistic and Richards formulations of the general 

 selectivity curve were fitted by using Excel (Microsoft, 

 Redmond, WA) routines (Tokai 2 ). These two selectivity 

 functions were chosen because of their relative simplic- 

 ity, their broad use over a range of different fisheries, 

 and the availability of estimation routines for their 

 parameters (Millar and Fryer, 1999). 



The Richards curve has the equation 



r(l) 



( exp(a+b, 

 (l + exp(a + 



bl)_ 

 bl) 



where r(l) is the probability that an individual of length 

 I attempting to pass through a mesh of given size will 

 be retained by it (Millar and Fryer, 1999); and a, b, and 

 5 are constants. The logistic curve is the special case of 

 this formulation, where 5=1. 



According to these models, the lobster length at 50% 

 retention (L 50 ) and the selection range {SR=L 75 -L 25 ) 

 are defined as follows: 



In 



0.5" 

 1 - 0.5' 



simplifying to L 50 = - — when 5 = 1, and 

 b 



: Tokai, T. 2002. Personal commun. Department of Marine 

 Science and Technology, Tokyo University of Fisheries, Konan 

 Minatoku, Tokyo 108, Japan. 



