FOGARTY and BORDEN: EFFECTS OF TRAP VENTING ON GEAR SELECTIVITY 



Table 2. — Catch per unit effort of American lobster in vented and nonvented traps for individual 

 fishermen. CTH indicates catch per trap haul; CTHSOD indicates catch per trap haul/set over day; the 

 subscripts L and S indicate the catch of legal ( s»78 mm CD and sublegal lobster, respectively. Data are 

 expressed in numbers of lobster. 



Interestingly, vented traps tended to consis- 

 tently catch more legal-sized lobster than control 

 traps (Tables 1,2). The overall mean CTH for legal 

 lobster was 0.945 in vented traps and 0.848 in 

 nonvented gear (Table 2). We attributed the trend 

 in lower legal catch in control traps to a saturation 

 effect where the probability of a lobster entering a 

 trap declines with increasing density within the 

 trap. In nonvented traps, sublegal lobster occupy 

 space which might otherwise be taiken by legal- 

 sized lobster. Direct evidence of catch density de- 

 pendence of this type in a trap fishery has been 

 demonstrated for two species of Cancer (Miller 

 1979). 



The well-established aggressive behavior of 

 lobster when held in confinement supports the 

 concept of a saturation effect for this species. Lob- 

 ster are characteristically solitary imder natural 

 conditions (Cobb 197 1; O'Neill and Cobb 1979) and 

 it is reasonable to assume that the presence of 

 lobster within a trap deters further entries. Al- 

 though relatively little is known of the trap- 

 related behavior of this species, there is an appar- 

 ent conflict between food (and/or shelter) seeking 

 behavior and avoidance of conspecifics. 



Krouse (1978) reported an increase in legal 

 catch in vented traps, supporting conclusions de- 

 rived in an earlier study conducted in Maine 

 (Krouse and Thomas 1975). Templeman (1939) 

 and Wilder (1945) had earlier demonstrated in- 

 creased legal catch rates in traps with increased 

 lath spacing. 



In the present study, the impact of crowding on 

 the legal catch was most pronounced in small 

 mesh (2.5 cm x 2.5 cm) wire traps (Fisherman F). 

 These traps retained extremely high numbers of 

 sublegals. Vented traps not only retained fewer 

 sublegal lobster, but caught substantially more 

 legal-sized lobster (Tables 1, 2). 



To further assess the effectiveness of the vented 



gear, we tested the hypothesis that the catch of 

 legal and sublegal lobster was independent of trap 

 type (vented vs. control). In two instances where 

 loss of trap resulted in an unequal number of ob- 

 servations, catch totals were adjusted by deleting 

 data from an adjacent trap to retain a balanced 

 design. Lost traps were replaced as quickly as pos- 

 sible. These analyses confirmed that significant 

 differences exist in the catch characteristics of 

 vented and control traps for combined data (Xi^ = 

 498.433; P<0.005) and for individual fishermen 

 (Table 1) with a single exception. 



Effect of Immersion Time 



The importance of incorporating soak time in 

 measures of CPUE has been emphasized in sev- 

 eral trap fisheries including that for the American 

 lobster (Thomas 1973; Skud^), the European lob- 

 ster, H. gammarus (Bennett 1974), the spiny lob- 

 ster ,Panulirus argus (Austin 1977), and the west- 

 ern rock lobster, P. longipes cygnus (Morgan''). 



The immersion (soak) time utilized by indi- 

 vidual fishermen is most often a function of the 

 total number of traps deployed and the daily haul- 

 ing capacity of the boat, although weather condi- 

 tions frequently interrupt hauling schedules. 

 Each fisherman typically has three or more sets of 

 gear which are hauled in rotation. 



Catch data were pooled and examined for the 

 effect of immersion time up to a maximum of 7 set 

 over days. Soak times of >7 days were omitted due 

 to excessive variability. The catch of legal lobster 



'Skud, B. E. 1976. Soak time and the catch per pot in an 

 offshore fishery for lobsters Qiomarus amencanus). Int. Cons. 

 Explor. Mer, Special meeting on population assessments of 

 shellfish stocks. No. 8, 25 p. 



^Morgan, G. R. 1976. Trap response and the measurement 

 of effort in the fishery for the western rock lobster. Int. Cons. 

 Explor. Mer. Special meeting on population assessments of 

 shellfish stocks, Contrib. 16, 18 p. 



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