KROUSE: EFFECTIVENESS OF ESCAPE VENT SHAPE 



Table l. — Lobsters caught with nonvented and various types of vented traps from July through 



November 1976. 



and vented traps (Table 1). Vented traps caught 

 fewer sublegal lobsters per trap-haul than non- 

 vented traps (t-test, P<0.01). 



The ratio of sublegal to legal lobsters did not 

 differ among the four types of vents (^-test, 

 P>0.1), with the exception of 61-mm circular 

 vents which caught fewer sublegals than horizon- 

 tal vents (P<0.01). As will be discussed later, the 

 61-mm hole is slightly oversize for a minimum size 

 of 81 mm CL, thus some smaller legal lobsters and 

 most shorts escape. Nevertheless this information 

 suggests that circular openings are as effective as 

 the rectangular vent ( Krouse and Thomas 1975) in 

 permitting escapement of short lobsters. 



To further assess the relative efficiencies of the 

 various vents, catch-effort values (numbers of 

 lobsters per trap haul set over day, CPUE) were 

 calculated and plotted for legal-sized and all-sized 

 lobsters combined for each vent type (Figure 2). 

 For this figure, 58- and 61-mm circular vent data 

 were pooled because of the small sample size and 

 similar catch values. Figure 2 graphically shows 

 that the CPUE for legal-sized lobsters was similar 

 for all vent types; however, for combined catches of 

 legals and sublegals, the CPUE for nonvented 

 traps was several fold greater. Thus, this indicates 

 that all traps tested were about equally efficient in 

 capturing legal lobsters; but, as to be expected, 

 nonvented traps caught substantial numbers of 

 short lobsters which probably would have escaped 

 from vented traps. Most importantly, these data 

 support an earlier conclusion that circular vents 

 select about the same size lobsters as do rectangu- 

 lar vents. 



Lobsters in Commercial Gear 



Catch data provided by a local lobsterman were 

 compiled according to the following categories of 

 gear: 1) end vented traps with a single circular 

 hole of 58 mm diameter (Figure IB); 2) side vented 

 traps with paired round openings of 58 mm diame- 



4.2 r 



3.9 



3.6 



3.3 



_l 



=) 3.0- 



< 



^ 2.71- 



a. 



< 2.4 



CE 



>- 2-1 



S 1.8 

 a. 



1.5 

 </) 



S 1.2 



Z .6 

 .3 



1.2 

 .9 

 .6 

 .3 



LEGALS and SUBLEGALS 



-I- HORIZONTAL VENT 



-O— NO VENT 



O CIRCULAR VENT 



A VERTICAL VENT 



2 3 



SET OVER DAYS 



Figure 2. — Comparison of the number of lobsters (legals only; 

 sublegal and legals combined ) per trap haul set over day for lobster 

 traps with rectangular (horizontal and vertical) and circular vents 

 (58 and 61 mm combined) and traps without vents. 



ter (Figure ID); and 3) two groups of nonvented 

 traps (one for nonvented traps fished in the same 

 trawl string with end vented traps and the second 

 for traps paired with side vented traps). Compari- 

 sons of the CPUE and the ratios of sublegals to 

 legals indicated that vented traps caught fewer 

 sublegal-sized lobsters than the corresponding 

 groups of nonvented traps (^-test, P<0.01) (Table 

 2). Higher CPUE values for vented traps show 

 that circular vents are at least as efficient if not 

 more effective in catching legal-sized lobsters 

 than nonvented traps (f-test, P<0.01). In an ear- 

 lier study Krouse and Thomas (1975) reported 

 that traps with 44.5 x 152.4 mm rectangular 

 vents were more successful in catching legal 

 lobsters than traps with smaller vents or no vents. 



427 



