EFFECTIVENESS OF ESCAPE VENT SHAPE IN TRAPS FOR 



CATCHING LEGAL-SIZED LOBSTER, HOMARUS AMERICANUS, 



AND HARVESTABLE-SIZED CRABS, CANCER BOREALIS 



AND CANCER IRRORATUS^ 



Jay S. Krouse^ 



ABSTRACT 



During 1976 a study was conducted to find an escape vent that would select similar sized lobsters as the 

 rectangular vent, yet retain Cancer crabs s90 mm carapace wddth. Analysis of the size composition of 

 research and commercial catches from experimental traps revealed that circular (58 mm in diameter) 

 and rectangular (44.5 x 152.4 mm) vents release shorts and retain legal lobsters ( 3^81 mm carapace 

 length) equally well, and decidedly more marketable-sized crabs were captured in traps with circular 

 vents. Length-width relationship shows that crabs 3=90 mm carapace width have lengths 3^58 mm, 

 thus precluding the possibility of marketable-sized crabs exiting through an opening 58 mm in 

 diameter. Escapement studies for lobsters confirm that with the present minimum legal size of 3^/i6 in, 

 a 58-mm diameter vent vnll select legals and allow most of the sublegals to escape. 



Accordingly, the Maine Department of Marine Resources recommends that either circular ( 358 mm 

 in diameter) or oblong (3=44.5 x 152.4 mm) escape vents be incorporated in all crab and lobster traps 

 along the Maine coast. 



Although rectangular escape vents are a very 

 beneficial type of savings gear for the lobster 

 fishery (Templeman 1939; Wilder 1945, 1948, 

 1954; Krouse and Thomas 1975; Krouse 1976), 

 this vent does not retain marketable-sized rock 

 crab, Cancer irroratus, and Jonah crab, C. 

 borealis. Since these commercially important crab 

 species are often caught incidental to lobsters, I 

 undertook the present study to find an escape 

 opening that would retain harvestable-sized crabs 

 and have similar fishing selectivities for the lob- 

 ster, Homarus americanus, as the rectangular 

 vent. 



In designing a trap to catch crabs and exclude 

 lobsters, Stasko (1975) observed in laboratory 

 tests that circular holes retained commercial- 

 sized crabs yet allowed small lobsters to escape; 

 however, the effectiveness of escape holes was not 

 tested in the field. Jow (1961) demonstrated the 

 advantages of circular escape openings in the trap 

 fishery for Dungeness crab, C. magister. 



In this paper I evaluate the relative efficiency of 



'This study was conducted in cooperation with the U.S. De- 

 partment of Commerce, National Marine Fisheries Service, 

 under Public Law 88-309, as amended, Commercial Fisheries 

 Research and Development Act, Project 3-153-R. 



^Maine Department of Marine Resources, West Boothbay 

 Harbor, ME 04575. 



circular and rectangular vents by examining data 

 from: 1) commercial and research catches com- 

 piled from vented and nonvented traps; 2) studies 

 of escapement from traps; and 3) certain mor- 

 phometric relationships of crabs and lobsters. 



METHODS 



From November 1976 through March 1977 a 

 commercial fisherman recorded and provided me 

 with catch data from traps with circular vents 

 (58-mm diameter) fished alongside traps without 

 vents. This experimental gear was arranged into 

 two groups with four trawls [series of six traps 

 spaced about 6 fathoms ( 11.0 m) apart with a sur- 

 face buoy at either end] per group. In each group 

 half the traps in a trawl had no vents, while the 

 remainder had either single (end of trap) or paired 

 (side of trap) vents depending upon the group (Fig- 

 ure IB, D). Every time the fisherman hauled 

 these traps he recorded the following information: 

 1 ) number of days traps were set between hauls; 

 and 2) number of lobsters &81 mm carapace 

 length, CL (keepers), and <81 mm CL (shorts) 

 caught in the vented and nonvented traps for each 

 trawl string. 



From July through November 1976, project per- 

 sonnel fished commercial lobster traps near 



Manuscript accepted July 1977 



FISHERY BULLETIN: VOL. 76, NO. 2, 1978. 



425 



