KROUSE and THOMAS: EFFECTS OF TRAP SELECTIVITY ON LOBSTERS 



four coastal areas. These boats were selected on a 

 nonrandom basis because not all vessel-captains 

 could or would accommodate us, nor could we 

 reallocate committed time from our ongoing sur- 

 veys of the natural lobster population and the 

 commercial catch. While aboard these vessels we 

 recorded the following: 1) numbers of sublegal and 

 legal lobsters for each trap haul; 2) carapace length 

 and sex of lobsters from a systematic sample of 

 the catch, along with the corresponding 

 measurements of lath spacings in these traps; 3) 

 time expended in actual fishing as well as fishing 

 time for each trap (number of set-over-days); 4) 

 whether the fisherman was hauling one trap at a 

 time (singles) or two attached traps with one buoy 

 (pairs), or three or more attached traps (trawls) 

 with two buoys, one at each end of the string; and 

 5) amount and kind of bait used. 



Carapace lengths were measured in millimeters 

 from the posterodorsal edge of the eye socket to 

 the posterior margin of the carapace. In most 

 cases, we attempted to measure all the lobsters in 

 every nth trap (depended on whether traps were 

 set as singles, pairs, or trawls); however, some- 

 times with two samplers, we were able to measure 

 and record all the lobsters in each of the total 

 number of traps hauled for the day. 



Length compositions of the catches for each 

 boat trip were used to calculate what we refer to as 

 retention curves. These curves are simply an ac- 

 cumulative percentage of the number of lobsters 

 by 1-mm carapace increments that were retained 

 in the systematic sample of the traps hauled, along 

 with measurements of the lath spacings of these 

 traps. Because lath spacings were not uniform for 

 each of the traps hauled per boat, the term "modal 

 spacing" was used to imply that at least a majority 

 of the traps per boat had a spacing more 

 frequently measured by us than any other. 



Samples from Research Gear 



Since 1968 we have recorded the carapace 

 length, weight, sex, condition (hard or soft shell, 

 lost appendages) of individual lobsters caught in 

 our research traps. Our research gear consisted of: 

 1) modified wooden traps, with plastic escape vents 

 of IV2, 1%, and 1% inches (Figure 1), and 2) 1 x 1 

 inch wire meshed traps especially designed to 

 catch sublegal lobsters. The modified commercial 

 gear was fished from July 1972 through 1973, while 

 the wire traps were used since 1968. 



We also conducted a trap escapement study 



Figure 1.— Modified commercial lobster trap equipped with a 

 plastic escape vent. 



whereby lobsters of known sizes were placed in 

 wooden traps with vents of IV4, IV2, 1%, and 2 

 inches. Because the heads or entrances were 

 sealed, any escapement should have been ac- 

 complished between the laths. Through a 2-wk 

 period traps were usually checked daily for es- 

 capement. 



Comparison of Samples from 

 Commercial and Research Gear 



Following the methodologies of Beverton and 

 Holt (1957), Pope (1966), and Gulland (1969), we 

 calculated selectivity curves which were based 

 upon carapace lengths of lobsters retained in the 

 commercial gear with modal lath spacings of IV4, 

 1%, and 1% inches. These data were proportioned 

 with the same range of lengths retained in the 

 1x1 inch wire meshed research traps. Both sets of 

 data, commercial and research, were weighted by 

 trap-haul-set-over-days (THSOD). These com- 

 parisons were from the same general area, but not 

 with the same groups of traps nor necessarily 

 during the exact same period of time. 



In addition, we used the cited methods to make 

 selectivity determinations from the modified 

 commercial traps that had specific lath spacings of 

 IV2, 1%, and 1% inches. These spacings were 

 proportioned with the data from the wire research 

 traps (1x1 inch mesh). In this case, the modified 

 commercial and wire research traps were fished 



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