EFFECTS OF TRAP SELECTIVITY AND SOME POPULATION 



PARAMETERS ON SIZE COMPOSITION OF THE AMERICAN LOBSTER, 



HOMARUS AMERICANUS, CATCH ALONG THE MAINE COAST- 



Jay S. Krouse and James C. Thomas^ 



ABSTRACT 



Information collected aboard commercial lobster boats along the Maine coast (1971-73) revealed, among 

 other things, high numbers of sublegal lobsters (<81 mm carapace length) being handled by fishermen 

 while sorting their catches. Throw-back ratios of illegal to legal lobsters (1.8 to 12.4:1) varied in 

 association with lath spacing. Traps with spacings of 1% to 1% inches accounted for markedly fewer 

 sublegals than those traps with 1'4- to lV2-inch spacings. 



Selectivity curves calculated for research traps with escape ports of V/2, 1%, and 1% inches and a trap 

 escapement study demonstrate that a spacing of 1% inches is large enough to allow escapement of most 

 sublegals yet small enough to retain legal lobsters. A regression of carapace length on carapace width 

 shows that only an insignificant percentage of legal lobsters could physically squeeze through a P4-inch 

 opening. Thus, results of this study led us to recommend that with a minimum legal length of 81 mm, 

 traps should have 1%-inch escape vents. 



While riding aboard commercial lobster boats 

 along the Maine coast (1971-73) to collect detailed 

 catch and effort information, we frequently ob- 

 served lobster fishermen sort and throw back from 

 their traps excessive numbers of sublegal lobsters 

 (<81 mm carapace length). When one considers 

 that Maine lobstermen presently haul their traps 

 more than 20 million times each year, the magni- 

 tude of this sorting becomes apprent. Lobstermen 

 not only lessen the efficiency of their fishing 

 operations by needlessly handling sublegal lob- 

 sters but they also inadvertently increase the lob- 

 sters chances of becoming a cull (missing claw[s]) 

 or a victim to predatory fish while descending to 

 the ocean floor (D. G. Wilder, pers. commun.), 

 which in either case represents an economic loss to 

 the industry. 



A solution to this detrimental fishing practice 

 became apparent to us after a cursory analysis of 

 data from our earlier boat trips revealed an in- 

 verse relationship between lath spacing and 

 numbers of sublegal lobsters. Templeman (1939) 

 and Wilder (1945, 1948, 1954) also reported the 

 same relationship based on size composition of 

 catches from traps of various lath spacings. 



'This study was conducted in cooperation with the Department 

 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 0457.5. 



Although these Canadian scientists have long ad- 

 vocated the use of wider latch spacings to allow 

 sublegals to escape, presently only Newfoundland 

 has a lath spacing regulation of 1% inches. 



Because of the management implications of this 

 association between lath spacing and size com- 

 position of the catch, we undertook this investiga- 

 tion to quantitatively assess this situation with 

 several independent approaches, namely: 1) selec- 

 tivity curves; 2) trap escapement study; and 3) 

 certain morphological dimensions of lobsters. 

 Certain facets of this study were also valuable in 

 corroborating some previously estimated popula- 

 tion parameters such as natural mortality rates, 

 sex ratios, and spawning stock structure and size. 



These analyses have become increasingly im- 

 portant because we have recommended raising the 

 legal minimum size from the present 81 mm (3'Vi6 

 inches) to 89 mm (3V2 inches) carapace length. 

 Then this study not only has application for the 

 present situation, but also provides pertinent in- 

 formation for management of lobsters in the fu- 

 ture. 



METHODS 



Samples from Commercial Gear 



From 1971 through 1973, we spent 21 days riding 

 aboard nine different commercial lobster boats 

 (three boats were sampled more than once) from 



Manuscript accepted December 1974. 

 FISHERY BULLETIN: VOL. 73, NO. 4, 1975. 



862 



