KROUSE and THOMAS: EFFECTS OF TRAP SELECTIVITY ON LOBSTERS 



60 70 80 



CARAPACE LENGTH (MM) 



Figure 5.-Selectivity curves for each of the three vent sizes (1%, 

 1%, and 1% inches) of the modified commercial traps compared to 

 wire (1x1 inch mesh) traps for 1972 and 1973. 



to the selectivity curves, all three vents would 

 prohibit the escapement of legal-sized lobsters. 

 However, the l^-inch curve falls closer to the 81- 

 mm line, thus demonstrating that this size vent 

 allows a greater percentage of sublegal lobsters to 

 escape than the 1%- or lV4-inch vents. 



Effects of Vent Size on Trap Efficiency 



Catches with modified commercial traps reveal 

 an inverse relationship between vent size and the 

 ratios of sublegal to legal lobsters (Table 3). 

 Overall catches with traps having a 1%-inch vent 

 always consist of more legal than sublegal lobsters 



Table 3.— Ratios of sublegal to legal lobsters captured with wire 

 (1x1 inch mesh) and modified commercial (IV2-, 1%-, and 1%-inch 

 vents) lobster traps, 1972 through 1973. Actual numbers of 

 sublegal and legal lobsters appear in parentheses. 



while this size composition is reversed in the 

 catches from traps with smaller vents. This 

 further substantiates our contention that exces- 

 sive handling of short lobsters in the lobster 

 fishery can be minimized with the addition of a 

 1%-inch vent to all lobster traps. 



Throughout this study, traps with 1%- and 1%- 

 inch vents not only retained fewer sublegal lob- 

 sters but seemed to capture proportionally more 

 legal-sized lobsters than did those traps with l'/2- 

 inch vents. To assess this situation, we calculated 

 separate catch-effort values (numbers of lobsters 

 per THSOD) for legal-sized and all-sized lobsters 

 combined for each of the three vent sizes (Figure 

 6). Indeed, our data indicate that traps with 

 larger vents (1% and 1% inches) are more success- 

 ful in retaining greater numbers of legal lobsters 

 than traps with smaller vents. However, because 

 of our limited field sampling, we cannot validly 

 conclude that this disparity in efficiency between 

 vents is conclusive evidence, but rather that our 

 data strongly suggest this possibility. 



tEGALS & SUBLEGALS 



2 3 



SET-OVER- DAYS 



Figure 6. -Comparisons of the number of lobsters (legals, 

 sublegals, and legals combined) per trap-haul-set-over-day for 

 modified commercial traps with vents of l'/2, 1%, and P4 inches 

 (1972-73). 



Body Proportions of Lobsters 



The escapement of lobsters of given sizes from 

 traps with varying lath spacings depends upon 

 certain morphological dimensions such as carapace 

 length, width, and height. We contend that the 

 width of the carapace is more important than the 

 height because we observed in the laboratory that 



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