Smith and Jamieson: Movement and mortality of Cancer magister 



145 



elude that our male estimate for S measures mainly 

 natural mortality of sublegal-sized males. We recog- 

 nize, however, that there is likely some degree of mor- 

 tality due to trapping and handling by fishermen. 

 Similarly, and in consideration that females are not 

 commercially fished, we are confident that our female 

 estimate for S also mainly measures natural mortality. 



Acoustic tagging 



From the means and standard errors of the movement 

 rate estimates (Table 4) we conclude that for both the 

 August and November 1986 releases of acoustically 

 tagged crabs, females tended to move about significant- 

 ly (P<0.01) more than males. Similarly, during Aug- 

 ust-October 1986 both males and females seemed to 

 move at rates about 3 to 4 times faster (P<0.01) than 

 those for November 1986-February 1987. We realize, 

 however, that our male results for the latter time 

 period are based on observations of just one crab. 

 Slower movement during winter for both male and 

 female crab is consistent with the poikilothermic habit 

 of marine invertebrates in that respiratory activity is 

 closely related to temperature. Our analysis of move- 

 ment based on the mark-recovery data also hints that 

 females are more mobile than males. 



We cannot conclude that our estimate of a faster 

 movement rate for females, relative to males, repre- 

 sents a general phenomenon for all Dungeness crab 

 populations. Our acoustic tagging results are possibly 

 peculiar to our study site, since the greater movement 

 rate for females is consistent with our interpretation 

 of the trapping and beam trawl results which suggest 

 that females tend to vacate the coastal inlets in search 

 of the exposed coast. Further, we caution that our 

 dispersal rate estimates for males and females from the 

 acoustic tagging data should be considered minimum 

 estimates because the archipelago in which the acous- 

 tically tagged crabs were released will tend to restrict 

 the potential for movement. Also, our assumption of 

 random dispersal from the point of release of an 

 acoustically tagged crab is probably too strict. Our 

 beam trawl and trapping data indicate that both males 

 and females might move in response to environmental 

 clues such as tides and currents; and our beam-trawl, 

 trapping, and mark-recovery data all suggest seaward 

 movement of females from Lemmens Inlet. 



These limitations to our interpretation of the acoustic 

 tagging data preclude us from obtaining precise rates 

 of population dispersal over time. However, we think 

 that there is some value in presenting approximate 

 dispersal rates since they can be compared with the 

 results of our analysis of movement and mortality, and 

 to some degree with Dungeness crab movement rates 

 that might be obtained from other regions. Conse- 



quently, the mean daily displacement rates (A) of 288 

 m/day and 419 m/day obtained for males and females, 

 respectively, suggest that after one year of random 

 dispersal, in the absence of geographical boundaries, 

 95% of males and females would be within radii of 9.5 

 and 13.9 km, respectively, of the point where they were 

 one year previous. These radii were determined from 



r 2 = -log e [p rt ]A 2 t 



(7) 



where r is the radius within which the proportion 

 1 - p rt of a randomly dispersing population is expected 

 after t days (Pielou 1977). These estimates of r agree 

 favorably with the results of our simultaneous analysis 

 of movement and mortality from our mark-recovery 

 data in that about 2% and 7% of male and female crabs, 

 respectively, were estimated to leave the study site dur- 

 ing the 18-month study period. The study site encom- 

 passes an area of about 5-10km radius around Tofino 

 (Fig. 1). 



Discussion 



The four methodologies we employed to assess male 

 and female Dungeness crab movement provided in- 

 sights at different levels of resolution and from differ- 

 ent perspectives. The methodologies were complemen- 

 tary and together allowed us to document a coherent 

 description of Dungeness crab movement near Tofino, 

 British Columbia. For example, acoustic tagging gave 

 us a general indication of the dispersal rates for in- 

 dividual male and female crabs (summer and winter) 

 which was consistent with the results of our simul- 

 taneous analysis of movement and mortality using 

 mark-recovery data. The former analysis suggested 

 that the dispersal rates would maintain 95% of the male 

 and female populations within about 10 km and 14 km, 

 respectively, of points of release, while the latter 

 analysis suggested that only about 2% of males and 7% 

 of females would escape the study site (about 10-15km 

 radius) during our 18-month study period. In addition, 

 both the beam-trawl and trap samples documented 

 seasonal changes in the relative distributions and abun- 

 dances of males and females which could not be gleaned 

 from the acoustic tagging and mark-recovery data 

 alone. 



Overall, our four sources of movement information 

 suggest that male Dungeness crab undergo only limited 

 movements within the local archipelago. There was no 

 evidence of migratory movement, but males were in- 

 ferred to move to shallower water (=10m) during sum- 

 mer, then to retreat to more sheltered habitat in 

 autumn. Others have observed, or inferred, similar be- 

 havior for this species. Stevens and Armstrong (1984) 



