KROUSE and THOMAS: EFFECTS OF TRAP SELECTIVITY ON LOBSTERS 



with those from the commercial and natural sur- 

 veys (Thomas 1973; Krouse 1973). The commercial 

 survey shows that about 6% of the yearly catch are 

 culls (one or both claws missing). Because most of 

 the legal catch is recently recruited, this may in- 

 dicate that frequent removal from traps of 

 sublegal lobsters is, in part, responsible for this 

 percentage of culls in the commercial catch. This 

 could occur because the claws of those sublegals 

 might grasp laths, knitted heads and parlor en- 

 trances, hands of fishermen, and the like. When 

 such lobsters are pulled from the traps by fisher- 

 men, the claws are occasionally broken off. 

 Another contributing factor might be that 

 sublegal and legal lobsters sometimes extrude 

 their claws through the lath spacings as the trap is 

 hauled aboard the vessel. In this way, claws could 

 be broken off. The design of the proposed "vent- 

 ed" trap, discussed later, takes this situation into 

 consideration. 



In order to evaluate more fully the possibility of 

 a higher natural mortality due to handling, we 

 used three independent approaches as follows: 



1. Our observations aboard vessels show that the 

 percentage of culls of sublegals is between 5 

 and 109c. This might indicate that natural 

 mortality has not increased due to handling 

 because of the similarity of the percentage of 

 culls in the sublegal and legal size range of 

 lobsters. Autotomy of the lobster could also 

 confound the percentage of culls; however, we 

 theorize that this particular percentage should 

 not be different from the sublegal to legal 

 sizes that we are studying. 



2. Another insight on the effect of natural mor- 

 tality would be the length frequencies of the 

 sublegal lobsters caught by research gear in 

 the sampling of the natural population 

 (Figure 3), as well as the length frequencies 

 from sampling aboard commercial vessels 

 (Figure 2), although gear selectivity is a factor 

 in this case. We should expect a higher mor- 

 tality due to handling to show a significant 

 decline in the number of sublegal lobsters as 

 the size range increases by 1-mm increments 

 from 70 (fully vulnerable size) to 81 mm (legal 

 minimum size). Then the number of lobsters 

 at, say, 80 mm should be less than those at 70 

 mm, not only due to the higher incidence of 

 handling this larger size, but also because of 

 the natural mortality that would occur 

 without handling. These numbers at the 



specific sizes do not show this decline that 

 could be attributed to a higher mortality due 

 to increased handling (Figure 3). 

 3. As a supplement to the incidence of handling 

 lobsters and the resultant natural mortality, 

 we feel that our observations on the storage of 

 lobsters in "pounds" (this procedure is 

 described in Thomas 1973) might give infor- 

 mation on the amount of natural mortality in 

 the natural population and that mortality due 

 to handling. The pound owners, stocking at 

 the rate of one to two lobsters per square foot, 

 tell us that a reduction of around 5% in 

 numbers is normal for legal lobsters stocked to 

 those reclaimed 3 to 5 mo later. Under these 

 adverse conditions of crowding and handling 

 in the pound as opposed to the situation in the 

 natural environment, we infer that the annual 

 natural mortality is low in the ocean (5 to 15%) 

 and that handling has a minimal effect. 



The loss in lobster pounds is sometimes 

 much higher than 5%, but in most of these 

 situations the higher loss can be attributed to 

 disease, adverse environmental conditions, 

 and escapement. 



Despite these speculative premises concerning 

 the negligible effects of handling on natural mor- 

 tality, the fishermen should still eliminate this 

 needless sorting of large numbers of sublegal lob- 

 sters to reduce: 1) the time spend sorting sublegal 

 from legal lobsters in traps, and 2) the eventual 

 number of culls in the legal catch. Culls not only 

 lessen the total poundage of the commercial catch 

 but possibly the growth rate of culls may be slower 

 than that of noncuUs; Stewart and Squires (1968) 

 suggest that molting of unduly stressed lobsters 

 may be inhibited. The section on selectivity will 



CARARkCE lENGTM (MM) 



Figure 3.-Length-frequency distributions of lobsters collected 

 with wire traps (1x1 inch mesh) at Boothbay Harbor (1972-73). 



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