178 



Fishery Bulletin 90(1). 1992 



Table 5 



Estimates of instantaneous mortality (Z) 

 and annual survival (S) for older age- 

 classes of Dungeness crab, Grays Harbor, 

 WA, and adjacent coast combined. Esti- 

 mates are for July-July, average for 

 several years. 



abundant in the Outer Harbor, where they reached 

 peak densities in the summer season. 



Mortality Estimated instantaneous mortahty rates 

 for age 0+ crab within Grays Harbor ranged from 1.6 

 to 3.5 yr- ', with a mean of 2.5 yr" ', corresponding to 

 an annual survival of 8.1% (Table 4). For older crab, 

 estimated mortality rates (Eq. 7) decreased to age 3 -t- , 

 then increased slightly between ages 3 -i- and 4 -i- (Table 

 5). These two results were combined to derive the 

 seasonal survival schedule (Table 6) used in the model. 



Gear and season comparisons 



The results of gear/season comparison simulations are 

 presented in Figures 6 and 7. These data show the 



300 



. 200 



100 



Unodjusted Loss 



tquivoleni Adull Loss 



n 



p 



C H P 

 Apr-Moy 



CUP 

 Jun-S€p 



C H P 



Ocl-Dcc 



CUP 



Jon- Mar 



Figure 6 



Entrainment rates of Dungeness crab by season and dredge 

 type for the Outer Harbor, by age-class, (upper) Unadjusted 

 losses; (lower) age 2 + equivalent losses. Dredge types: C = 

 clamshell, H = hopper, P = pipeline. Age-classes as in Fig- 

 ure 5. 



strong contrast between the pipeline and clamshell 

 dredges: the clamshell dredge has negligible impact. 

 Comparing the unadjusted losses (Eq. 3) with age 2 -i- 

 equivalent losses (Eq. 4) shows the relative unimpor- 

 tance of -(- crab. Also notable are the high age 2 -i- 

 equivalent losses in the Outer Harbor during summer 

 and fall, when there are concentrations of age 1 + and 

 older crab in this area (Fig. 5). 



Impact estimates 



Calculations of total age 2-i- equivalent loss (Eq. 5) 

 for the two project scenarios are shown in Figure 8. 

 As expected, Scenario 1 (full use of the pipeline 

 dredge with confined disposal) shows higher losses 

 than Scenario 2. For both scenarios, a large part of 

 the total loss occurs during the June-September sea- 

 son, due to large volumes being dredged in the Outer 

 Harbor where older crab are concentrated at this time. 

 The results indicate strong year-to-year variation in 



