FISHERY BULLETIN; VOL. 73, NO. 4 



lobsters, attempting to escape through different 

 lath spacings, would twist on their sides when en- 

 countering a tight fit between laths. This, coupled 

 with the fact that the width is always smaller than 

 the height for any carapace length, led us to the 

 opinion that the relationship between carapace 

 length and width in association with lath spacing 

 is the important consideration for gear selectivity 

 studies. We calculated the regression of carapace 

 length (X) on width (Y) for 217 lobsters (114 

 females and 103 males) by the method of least 

 squares. The calculated equation was Y = -4.367 

 + 0.649 A' (Figure 7). Data for sexes were com- 

 bined because analysis of covariance on the 

 regression coefficients (Steel and Torrie 1960) 

 showed that carapace length-width ratios of males 

 and females did not differ significantly. 



According to this relationship, lobsters at the 

 minimum legal size of 81 mm carapace length 

 would be expected to have a mean carapace width 

 of 48.2 mm + 0.18 with individual widths within 

 the 95% prediction interval ranging from 45.6 to 

 50.9 mm. The magnitude of these measurements 

 relative to a rV4-inch (44.5-mm) lath spacing sug- 

 gests that only a very small percentage of legal- 

 sized lobsters might escape through that size vent. 



We should mention that some compression of 

 the shell, particularly if the lobster is newly molt- 

 ed, is possible as a lobster struggles to get through 

 the lath spacing of a trap. However, based upon 

 our laboratory observations, we would not expect 

 this compression to exceed 2 to 3 mm for soft- 

 shelled lobsters (1 to 2 wk since ecdysis) and 1 mm 

 for a hard-shelled lobster. Because this soft- 

 shelled condition is of rather brief duration and 



40 



1-75" ! " M.n,m«m lagal i,i. 



70 



EfO 85 



CARAPACE LENGTH (MM) 



90 



the frequency of trap hauls is greatest during the 

 shedding season (a shorter period of time for es- 

 capement), these situations should minimize es- 

 capement. 



Trap Escapement Study 



Retention curves, based on the escapement of 

 lobsters of known lengths from traps with lath 

 spacings ranging from IV4 to 2 inches, graphically 

 display the pronounced effect escape vent size has 

 on lobster escapement (Figure 8). Retention of 

 sublegal lobsters was high for the 1*4- and 1 '/2-inch 

 traps while most sublegals were able to escape 

 from traps with the 1%- and 2-inch vents. With the 

 present minimum size of 81 mm (3-^/i6 inches), a 

 2-inch vent would be unsatisfactory as many legal 

 lobsters could escape, whereas escapement of 

 legals through a 1%-inch vent would be extremely 

 minimal. Although the curve for the r'A-inch vent 

 did show some escapement, we believe this es- 

 capement is exaggerated by the methodology 

 (plotting midpoints) employed in the derivation of 

 this curve. This contention is further substantiat- 

 ed by the fact that only one of seven lobsters with 

 a carapace length of 82 mm escaped and there was 

 no escapement for lobsters larger than 82 mm. 



60 



70 80 90 



CARAPACE LENGTH (mm) 



Figure 7.-Carapace length-width relationship for lobsters with 

 95% confidence and prediction intervals. 



Figure 8. -Retention curves for lobsters placed in modified com- 

 mercial traps with VA-, VA-, and 1%-, and 2-inch vent dimensions. 



870 



