FISHERY BULLETIN; VOL. 76, NO. 2 



lobsters with 81 mm CL had <58 mm CH; 2) about 

 half those lobsters with 84 mm CL had <58 mm 

 CH; and 3) lobsters >90 mm CL had ^58 mm CH. 

 Based on this relationship alone, it appears that 

 many lobsters ranging from 8 1 to 89 mm CL would 

 be able to squeeze through a 58-mm diameter hole; 

 however, this is refuted by the previous sections on 

 the commercial and research catches of lobsters 

 with circular vented traps and the following dis- 

 cussion of escapement studies. Lobster escape- 

 ment through a round opening cannot be accu- 

 rately determined by carapace height alone since 

 this measurement excludes the walking legs 

 which contribute to the lobster's overall height or 

 depth. Whether or not a lobster is successful in 

 passing through a round hole will be determined 

 not only by the lobster's greatest transverse di- 

 mension (carapace height plus protruding legs) 

 but also by the lobster's ability to maneuver 

 through a tight opening. 



Obvious limitations with the aforementioned 

 morphometric relationship caused me to seek an 

 alternate approach to assess escapement. Thus, I 

 decided to determine the largest size lobster that 

 could be manually passed through a 58-mm 

 diameter hole. Lobsters 81 mm CL passed through 

 the hole rather easily following careful manipula- 

 tion of the walking legs and 82-mm CL lobsters 

 required considerable force, often causing bodily 

 harm, while larger lobsters ( >82 mm CL) could 

 not pass through the opening. 



Patterns of escapement for lobsters ranging 

 from 78 to 84 mm CL from traps with 58-, 60-, and 

 61-mm diameter vents varied decidedly as de- 

 picted by retention curves in Figure 6. Only the 

 58-mm vent retained all legal-sized lobsters and 

 still had reasonably high escapement of sublegals; 

 whereas, the other vents which were merely 2 or 3 

 mm larger allowed legal-sized lobsters to escape. 

 These data emphasize the importance of accu- 

 rately producing the 58-mm opening, else the 

 vent's desired effect will be lost. 



Crabs 



Carapace width-length relationships for C. 

 borealis and C. irroratus graphically show that 

 crabs >90 mm CW (commercially harvested size) 

 have carapace lengths (dimension limiting es- 

 capement) which exceed 58 mm (Figures 7, 8). 

 Accordingly, commercial-sized crabs of either 

 species cannot egress through a circular opening 

 58 mm in diameter. In fact, if the vent diameter 



77 78 79 80 81 



CARAPACE LENGTH (mm) 



83 84 



Figure 6. — Retention curves for lobsters placed in lobster traps 

 with circular vents of 58, 60, and 61 mm in diameter. 



were increased to as large as 65 mm (certainly, an 

 over estimate) to accommodate an upward shift in 

 the lobster minimum size (Maine Department of 

 Marine Resources recommends an increase from 

 3^/i6 to 3y2 in CL by Vi6-in increments annually 

 over a 5-year period) this would have little or more 

 likely no effect on catches of marketable crabs. 



RECOMMENDATIONS 



In view of the findings of this study and past 

 investigations (Krouse and Thomas 1975; Krouse 

 1976), all lobster and crab traps fished in Maine 

 waters should have a rectangular escape vent not 

 less than 1.75 in (44.5 mm) by 6 in ( 152.4 mm) or at 

 least two circular escape vents not less than 2.28 

 in (58 mm) in diameter. To insure maximum es- 

 capement of sublegal lobsters, vents should be in- 

 stalled next to the sill on the side or end of the 

 trap's parlor section. 



Although fishermen should certainly have the 

 option to fabricate their own vents, provided that 

 the prescribed dimensions are adhered to, the use 

 of synthetic, prefabricated vents is highly recom- 

 mended (Krouse and Thomas 1975). Recently, a 

 plastics manufacturer assured me that vents could 

 be produced and retailed for about 200 each. At 

 this low price and with today's high price of laths 

 (about 50 each), if a synthetic vent replaces two 

 laths every 3 yr, then after 6 yr the original cost of 



430 



