FISHERY BULLETIN: VOL. 73, NO. 4 



demonstrate the potential benefit of proper lath 

 spacing. 



Retention Curves 



The accumulative length frequencies by 1-mrn 

 increments from two selected boat trips reflect a 

 characteristic sigmoid curve (Figure 4). Snedecor 



lOOr 



70 78.0 80 95" 



CARAPACE LENGTH (MM) AT CAPTURE 



Figure 4. -Retention curves for two commercial boat trips (refer 

 to boat trips 4 and 5 in Tables 1 and 2) in Boothhay Harbor (1972). 

 Boat trips 4 and 5 had modal vent spacings of V/t and 1% inches, 

 respectively. 



(1956) and others caution that although these 

 curves are characteristic of normal distributions, 

 other types of distributions could result in a sig- 

 moid curve. For our purposes, we assume that 

 these data represent normal distributions. 



These curves are interesting in themselves 

 because the 50% accumulative point demonstrates 

 the influence of different lath spacings on the size, 

 composition of the catch. For instance, boat trips 4 

 (lV4-inch vent) and 5 (1%-inch vent) had 50% ac- 

 cumulative points of 75.6 and 78.0 mm, respec- 

 tively; thus demonstrating that traps with IVi-inch 

 vents are more selective for smaller lobsters (85% 

 of catch ^81 mm) than traps with 1%-inch vents 

 (70% of catch <81 mm); while we are not advocat- 

 ing using these curves in place of selectivity 

 curves, however we do suggest that these "reten- 

 tion curves" could be used as a quick, preliminary 

 approximation of the influence of different lath 

 spacings (gear selectivity) on the size composition 

 of the catch in a trap fishery. 



Selectivity Curves 



Based upon the length composition of our 

 catches with modified commercial traps equipped 

 with escape ports (plastic vents) of IV2, lys, and 1% 

 inches and the 1x1 inch wire meshed traps, we 

 calculated selectivity curves in accordance with 

 the methodology of Beverton and Holt (1957). 

 These catch data by 1-mm increments were 

 weighted by THSOD and then the resultant values 

 for each of the three vents (lath spaces) were 

 proportioned with those of the wire traps over the 

 same range of carapace lengths. 



Traps with the same lath spacings had similar 

 selectivity curves for the 1972 and 1973 catches 

 while conspicuous differences are evident between 

 the various size vents (Figure 5). In both years the 

 lV2-inch vent was selective for the smaller sizes 

 (50% retention ranged from 68.2 to 68.6 mm 

 carapace length), the 1%-inch vent for the inter- 

 mediate sizes (50% retention ranged from 71.4 to 

 73.5 mm carapace length), and the P/4-inch vent for 

 larger sizes (50% retention ranged from 75.4 to 78.8 

 mm carapace length). Contrary to most selectivity 

 studies the important consideration in this study 

 is not the mean selection length (50%^ point at 

 which half the lobsters escape and half are re- 

 tained); but rather, the proximity of the curve to 

 the minimum legal size (81 mm carapace length) 

 and whether or not the 100% retention point occurs 

 below or above the minimum legal size. According 



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