Wainwright et al : Effects of dredging on a crab population 



177 



2000 



Ouler Hotbor 



ssrw ssfw ssfw ssfw ssfw ssrw ssrw 



1983 1981 1985 1986 1987 1988 Meon 



A 1 5000 



1500 



1000 



I 



,Ofl=.^,D,g,...il,lL 



.BI. 



ssrw ssrw ssrw ssrw ssrw ssrw ssrw 



1983 1984 1985 1986 1987 1988 Meon 



Figure 5 



Seasonal abundance (catch per hectare) of Dungeness crab 

 in the Outer and Inner Harbor strata of Grays Harbor, Wash- 

 ington, by age-class. Solid bars, age 0-r; white, age 1 + ; 

 hatching, age > 1 -r . 



Once project scheduling was determined, predictions 

 of total crab loss were needed, which we calculated by 

 simulating entrainment for planned construction sce- 

 narios. The scenarios we have used for calculating crab 

 losses reflect the project as planned in 1987 (Table 3). 

 There was some conflict between project costs and crab 

 protection, particularly regarding the tradeoff between 

 using gear that is economically efficient (hopper and 

 pipeline) and that which minimizes loss (clamshell). 

 Throughout most of the estuary, the efficiency of the 

 hopper dredge makes alternatives uneconomic. In cer- 

 tain areas of the Inner Harbor, the pipeline dredge is 

 economically most efficient but results in high post- 

 entrainment mortality. The alternative dredge in those 

 areas is a clamshell, which is generally more costly. To 

 better evaluate this tradeoff, two scenarios are con- 

 trasted. Scenario 1 includes full use of a pipeline dredge 

 where it is most effective; in Scenario 2, a clamshell 

 dredge is substituted where feasible. Table 3 shows 

 volumes dredged under each scenario by gear type, 

 location, and season. 



As initially planned, construction was to occur over 

 two calendar years, extending through seven seasons. 



To simplify calculations, we compressed the project into 

 a single model year (from spring of a given calendar 

 year through winter of the next), and calculated en- 

 trainment and losses for each scenario separately based 

 on each of the six years of survey-based crab abundance 

 estimates. This produced a set of 12 (six years by two 

 construction scenarios) model runs. 



Because the project was revised in several ways since 

 these calculations were made, results presented here 

 do not reflect actual expected losses resulting from the 

 project, and are presented only to illustrate the method. 



Results 



Population parameters 



Age-class abundance Densities of crab in the Inner 

 and Outer Harbor strata varied considerably among 

 years and seasons (Fig. 5). Average seasonal total den- 

 sity ranged from 73 ha-^ to 13,000 ha ^ Age O-i- crab 

 were most abundant in 1984, and were usually more 

 abundant in the Inner Harbor. Older crab were more 



