STEVENS AND ARMSTRONG: CANCER MAGISTER IN GRAYS HARBOR 



M J 



SOND'JFMA 



1980 1981 



M J 



FIGURE 9.— Growth rates of Cancer magister from Grays Har- 

 bor, Wash. Points represent log 10 mean dry weight calculated 

 from mean width by regression equation. Straight lines (fitted 

 visually) show slope of growth curve (k) for summer and winter 

 growth stanzas of 1980 (0+), 1979 (1 + ), and 1978 (2 + ) year 

 classes. Growth stanzas arbitrarily separated at 28 August 1980. 



tribution greatly overlapped that of the 0+ age 

 class, age 1+ crabs showed proportionally less use 

 of low-salinity stations such as 6, 8, and 9. Al- 

 though this group was also abundant near eel- 

 grass beds, they were restricted to the subtidal 

 channels and showed only intermittent use of 

 mudflat areas during high tides, whereas many 

 crabs of the 0+ group remained in the littoral zone 

 at low tide. 



The 2+ age group, consisting of sexually mature 

 crabs (Poole 1967), was abundant only at the outer 

 estuary stations (1, 3, 4, 6, and 7). Many crabs 

 probably migrate out of the harbor before reaching 

 age 3 + . This hypothesis is supported by the scar- 

 city of age 3+ crabs east of station 3 and the total 

 absence of gravid females from trawls taken in the 

 estuary, although many trawls were made during 

 the spawning season (October-March). Appar- 

 ently, most mature females leave the harbor to 



spawn. Stressful salinity and temperature, high 

 larval predation in the estuary, or inadequate lar- 

 val food supplies might have created selection 

 pressures for spawning females to seek water 

 offshore with the proper environmental conditions 

 for higher egg and larval survival. 



Crab Population Estimates 



A more important question than that of local 

 crab densities at several stations is that of total 

 population abundance throughout the estuary, de- 

 termined for different seasons and age classes. 

 Such a calculation is of interest in order to 

 gauge 1) the potential use of the estuary by the 

 species and the 0+ age class in particular; 2) the 

 theoretical contribution made by the estuarine 

 population to the commercial fishery; and 3) the 

 potential impacts of estuarine development (e.g., 

 dredging and landfill) on resident populations. 

 The former two points are addressed in this 

 discussion. 



Trawl Efficiency Estimates 



In order to extrapolate crab density values at 

 each station to abundance within the correspond- 

 ing stratum, some measure of trawl efficiency was 

 needed. Examinations of mudflat areas (stations 5 

 and 14) in May of 1981 and 1982 showed very high 

 densities of 0+ age crabs, ranging from 1 to 5 crabs 

 m -2 . This estimate was conservatively reduced to 1 

 crab m - 2 , and we assumed that only 50% of the 

 available estuarine bottom (Fig. IB) was utilized 

 by early instars (excluding the inner estuary and 

 perimeter which had lower salinities and had pro- 

 duced few or no crabs of this age group). This 

 "corrected" density of 0.5 crabs m~ 2 (5,000 crabs 

 ha -1 ) was about 30 times the mean summer den- 

 sity of age 0+ crabs at station 5 (162 crabs ha- x ) as 

 estimated by trawl; thus trawl efficiency in that 

 season was about 0.033 (more recent studies have 

 shown early instar densities to equal or exceed 10 

 rrr 2 in 1983; D. Armstrong, unpubl. data). In 

 winter and spring, 0+ age group crabs were large 

 enough to be sampled more effectively, but proba- 

 bly not as effectively as larger crabs; thus a factor 

 of 0.25 was used in both of those seasons. An effi- 

 ciency factor of 0.5 was applied to all other age 

 groups, in accordance with Gotshall (1978a). 



Abundance Calculation 



For the nine strata of Grays Harbor sampled by 



479 



