STEVENS AND ARMSTRONG: CANCER MAGISTER IN GRAYS HARBOR 



TABLE 6. — Comparison of Cancer magister densities in Grays Harbor, Wash, (this 

 report); Humboldt Bay, Calif. (Gotshall 1978a); and San Francisco-San Pablo Bay, 

 Calif. (Orcutt et al. 1975, 1976; Orcutt 1977). Data are not corrected for gear efficiency. 



'Distance estimated as 50 m/min. 



2 Area estimated as distance (given) x % (headrope length). 



Growth 



Dry weight increased 282 times between first 

 instar (0.2 g) and sixth instar (5.7 g) during the 

 first year. Other authors have not presented 

 growth data as changes in weight, but rather as 

 increases in carapace width. Crabs in Grays Har- 

 bor grew from about 7 mm to 50+ mm cw during 

 1980-81, which is similar to values reported by 

 Cleaver (1949) and Butler (1961). However, Poole 

 (1967) concluded that crabs in Bodega Bay, Calif., 

 reached 75 mm (range 50-100 mm) by 1 yr after 

 metamorphosis. This would represent fairly rapid 

 growth, but close to the upper limits of crab growth 

 rates in Grays Harbor. 



In contrast to Grays Harbor, Tasto (1983) stated 

 that juvenile crabs spend only 1 yr in San Pablo 

 Bay, and reach 100 mm by the end of that time 

 (twice the growth rate of ocean crabs and Grays 

 Harbor crabs). He concluded that the estuarine 

 population was a single year class and was almost 

 completely replaced by a new year class each 

 spring, a situation very different from Grays Har- 

 bor where at least three year classes are present 

 constantly The San Francisco data may have been 

 misinterpreted, perhaps caused by use of collect- 

 ing gear (mostly ring nets) that selected larger 

 crabs and resulted in a frequency mode near 100 

 mm cw that may have actually represented older 

 1+ age group crabs. 



Unfortunately, growth data are not available for 

 0+ age Dungeness crabs that metamorphose di- 

 rectly offshore for comparison with estuarine 



populations. Presumably, colder bottom-water 

 temperatures offshore (8°-10°C) would cause 

 metabolic, growth, and general energetic depres- 

 sion of these animals relative to rates in warmer 

 (14°-18°C) estuaries. Studies of offshore juvenile 

 populations are much needed in this regard. 



Importance of Grays Harbor to 

 Commercial Fisheries. 



The potential contribution of Grays Harbor to 

 the commercial landings of Cancer magister was 

 calculated by assigning various mortality rates to 

 the 1980 year class for a period of 3.5 yr, i.e., until 

 recruitment to the fishery. Jow (1965) estimated 

 annual natural mortality of adult male crabs to be 

 15% per year (M = 0.165, exponential). Mortality 

 rates for juveniles are unknown, so we have as- 

 sumed a range of 0.5-0.8. From an initial popula- 

 tion (N ) of 28.9 million juvenile crabs in summer 

 of 1980, the number surviving 3.5 yr (N) was cal- 

 culated from the equation 



N = N e- Zt 



where z represents the annual mortality rate and 

 t is the time interval. Values of 2 used were 0.8 for 

 the first half year it = 0.5), 0.5 for the second half 

 year, and 0.2 (as above) for the remaining 2.5 yr 

 necessary to reach legal size, assuming crabs 

 enter the fishable population at that age, as sug- 

 gested by Cleaver (1949). At these mortality rates, 

 about 9.2 million adult crabs might remain by 



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