KROUSE: EFFECTIVENESS OF ESCAPE VENT SHAPE 



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Q. 36 

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 a: 3 3 



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UJ 2 1 



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S 1.8 



2^ 15 



1.2 



9 



6 



3 



12 3 4 5 



SET OVER DAYS 



Figure 4. — Comparison of the number of rock crabs ( 390 mm 

 carapace width i per trap haul set over day captured with non- 

 vented and circular and rectangular (horizontal and vertical) ven- 

 ted lobster traps. 



over days in Figure 4, particularly for circular and 

 nonvented traps where escapement could only re- 

 sult via the entrances, vividly demonstrate the 

 crab's ability to escape as trap soak time is in- 

 creased. Evidently, after the voracious crabs be- 

 come satiated by eating the trap's bait, which fre- 

 quently occurs in 1 or 2 days during the summer, 

 the trap loses its attractiveness and crabs try to 

 escape. Therefore, crab fishermen can maximize 

 their catches by hauling their traps daily, particu- 

 larly during periods of high catches. Contrasted to 

 declining crab catches with greater soak times are 

 lobster CPUE values which increase until 4 or 5 

 set over days, after which catches begin to di- 

 minish (Figure 2). Similar trends in CPUE data 

 for commercial catches have been reported by 

 Thomas (1973). Thus it appears that crabs are 

 more adept at escaping from traps than lobsters. 



Escapement and Morphometric Studies 



Lobsters 



horizontal and vertical vented traps than in the 

 circular and nonvented traps. Actually, horizontal 

 and vertical vents, unlike circular and nonvented 

 traps, also permit harvestable crabs to escape, re- 

 sulting in reduced catches of keepers. Con- 

 sequently, the proportion of nonkeepers to keepers 

 is markedly greater for horizontal and vertical 

 holes. 



As evidenced by the aforementioned catch data, 

 selectivity features of horizontal and vertical rec- 

 tangular vents are similar; however, compared to 

 circular vents they are unsatisfactory for catching 

 large crabs. Prior to field testing, guided by opin- 

 ions of some fishermen and our own thoughts, it 

 seemed plausible that a vertically positioned rec- 

 tangular opening (Figure lA) might inhibit es- 

 capement of those crabs with carapace length ex- 

 ceeding the vent's width (smallest dimension). Of 

 course, this was predicated on the assumption that 

 when a crab encounters such a narrow upright 

 opening it will only attempt to egress in a horizon- 

 tal plane and will not tilt the body diagonally. 

 However, laboratory observations and size com- 

 position of catches in traps with vertical vents 

 indicate crabs will readily turn on end or side to 

 exit. 



Prior to this, only escapement through the vent 

 itself has been discussed; this certainly does not 

 preclude escapement through entrance heads. 

 Diminishing CPUE values plotted against set 



Passage of lobsters through a round hole is re- 

 lated to the lobster's carapace height (greatest 

 cross-sectional dimension) relative to hole diame- 

 ter. Figure 5 shows that: 1) most legal-sized 



95% 



Prediction 

 Intervals 



85 90 



LENGTH (mm) 



Figure 5. — Carapace length-carapace height relationship for 

 lobsters with 95% confidence and prediction intervals. 



429 



