Smith and Jamieson: Movement and mortality of Cancer magister 



143 



100 -i 



VIRTUAL ENTRY RATE 



CATCH RATE 

 / (n.10 trap"'-d"' ) 



~~\ — I — i — I — I — r 



MAMJJASO 

 1985 



DATE 



Figure 4 



Two indices of abundance for female Dungeness crabs 

 > 145 mm CW in zone 6, where females were most abundant 

 (Table 2). The virtual catch rate is the rate at which females 

 would enter a trap if entry rates were not modified by changes 

 in bait effectiveness over time and agonistic interactions 

 among crabs (see Smith and Jamieson 1989a). 



Inlet continued to decline, suggesting seaward move- 

 ment. The increase in female abundance in September 

 1986 where the main channel out of the study site 

 meets the open coast (Fig. 3e) also suggests seaward 

 movement of females. These females were inferred to 

 be 3-year-olds because they were mainly in the normally 

 distributed instar with a mean carapace width of 

 137 mm and a standard deviation of 9 mm (Smith and 

 Jamieson 1989c). 



The relative abundance of larger females in the study 

 site was assessed by trap sampling. Two indices of 

 abundance obtained from commercial trap samples 

 (Fig. 4), show that females were most readily caught 

 in spring, perhaps because they forage more actively 

 after a winter of incubating eggs. The highest observed 

 abundance of the larger females (>145mm CW) in 

 spring 1985 and 1986 (Table 2) was in the main chan- 

 nel out of the study site. This is also where (presumably) 

 3-year-old females in the 137 mm instar were collected 

 in abundance by beam trawl in September 1986. This 

 is consistent with the suggestion from beam-trawl 

 sampling of seaward movement from local inlets as 

 females mature. 



Analysis of movement and mortality 



Our estimate of the catchability coefficient q for fe- 

 males of 0.198 x 10~ 5 recoveries per trap haul was 

 about one-seventh that for males (0.138 x 10~ 4 recov- 



eries per trap haul), indicating males were much more 

 readily caught than females by commercial traps. This 

 difference is mainly due to commercial traps being 

 more efficient at retaining larger crabs which are 

 predominantly males. Consequently it is apparent in 

 Table 3 that our transfer proportion estimates for 

 females are much less confidently made than those for 

 males because of the much smaller number of tagged 

 females recovered. 



The proportional transfer rates for males do not sug- 

 gest any persistent directional movement. We note that 

 there is little transfer of crabs between Indian Island 

 (zone 4) and Browning Passage (zone 5) when compared 

 with the apparent mixing among zones 1, 2, 3, 5, and 

 6, so the movement of tagged crabs through the deeper 

 water between Browning Passage and Indian Island 

 appears limited. Male crabs appear to vacate middle 

 and lower Lemmens Inlet (zones 2 and 3) at propor- 

 tional rates of about 0.22 and 0.40 per month with 



