FOGARTY and BORDEN: EFFECTS OF TRAP VENTING ON GEAR SELECTIVITY 



where Cj is the cumulative catch on days, C^is the 

 asymptotic catch, and R is the net retention rate 

 assuming constant availability. The term d is 

 dependent on not only the physical holding capac- 

 ity of the trap but on any behavioral interactions 

 which serve to limit the catch. The asymptotic 

 catch will be reached when ingress is balanced by 

 escapement. 



Parameters of the model were estimated by non- 

 linear least squares (Hartley 1961). The trend in 

 greater legal catch in vented gear was reflected in 

 the slightly higher estimate of C^ in vented traps 

 (Table 3). The substantially lower asymptotic 

 catch level for sublegal-sized lobster in vented 

 gear clearly demonstrated the effectiveness of 

 these traps. Munro ( 1974) stressed the importance 

 of escapement in determining saturation levels in 

 fish traps. 



This model may also be used to standardize ef- 

 fort to a common soak time. Adapting the ap- 

 proach of Sinoda and Kobayasi ( 1969) and Caddy,*' 

 weighting coefficients are given by 



1 — exp(— ills) 



u) = ^^ — 



1 — exp(—Rs*) 



Table 3. — Coefficients and associated standard errors for the 

 model Cs = C^i^ — exp( -Rs)] relating catch per trap haul and 

 soak time in vented and control traps for legal- ( ss78 mm CD and 

 sublegal-sized lobster. 



CARAPACE LENGTH (t 



FIGURE 3. — Size-frequency distribution of American lobster col- 

 lected in vented and control traps in Narragansett Bay-Rhode 

 Island Sound (1976-77). 



where s* is the standard soak time. The total effec- 

 tive effort (/,jj,) is then the product of nominal 

 effort (trap hauls) and the weighting coefficient 

 (Caddy see footnote 6) 



s 

 Aot = ^/"sW 



and the standardized CPUE is given by the catch 

 divided by /'(„, . Adjustment for variable soak times 

 should greatly improve the precision of catch ef- 

 fort data used in surplus yield modelling. 



Size Selectivity 



Carapace length (CL) measurements were ob- 

 tained for a sample catch of 2,943 lobster retained 

 in the experimental traps. The reduction in the 

 sublegal catch retained in vented gear was most 

 pronounced for lobster <75mmCL(Figure3). Size 

 selection for lobster >75 mm CL was virtually 

 identical in vented and control traps. The mean 



'Caddy, J. D 1977. Some considerations underlying defini- 

 tions of catchability and fishing effort in shellfish fisheries, and 

 their relevance for stock assessment purposes. Int. Cons. 

 Explor. Mer, Shellfish and Benthos Committee, CM. 1977/K;18, 

 21 p. 



CARAPACE 



FIGURE 4. — Retention curves generated for vented and control 

 traps for American lobster collected in Narragansett Bay-Rhode 

 Island Sound (1976-77). 



size of lobster caught in non vented traps (75.20 

 mm) and vented gear (78.99 mm) were sig- 

 nificantly different (t = 12.856; P<0.01). 



Retention curves (Krouse and Thomas 1975) 

 constructed for vented and control traps clearly 

 reflect the differences in the retention characteris- 

 tics for each trap tjrpe (Figure 4). The cumulative 

 retention points for each curve at the Rhode Island 

 minimum legal size at the time of this study (78 

 mm CL) were 56.0% and 69.5% for vented and 

 control traps, respectively. 



929 



