172 



Fishery Bulletin 90(1). 1992 



The work we describe here ap- 

 plies an equivalent adult loss 

 model (the "Dredge Impact 

 Model" or "DIM") to assessing 

 entrainment loss to the Dunge- 

 ness crab Cancer magister Dana 

 fishery in and around Grays Har- 

 bor, Washington. The Grays 

 Harbor navigation channel (Fig. 

 1) extends from the harbor 

 mouth to the city of Aberdeen, a 

 distance of about 25 km. The U.S. 

 Army Corps of Engineers cur- 

 rently removes an average of 1.2 

 million m^ of sediment annually 

 from the channel during main- 

 tenance dredging. To improve 

 accessibility for deep draft ves- 

 sels, the Corps planned to vnden 

 and deepen the channel by re- 

 moving about 8.7 million m^ of 

 material over a two-year period 

 (McGraw et al. 1988). Based on 

 results and predictions of DIM, 

 the Corps changed their original 

 dredging program by modifying 

 gear, volume dredged, and location/season combina- 

 tions to minimize impact on crab within operational 

 constraints (including weather and protection of other 

 resources). Project construction took place throughout 

 1990, ending in January 1991. This paper extends an 

 initial analysis (Armstrong et al. 1987), incorporating 

 two additional years of biological data and providing 

 a more thorough analysis of year-to-year variation. The 

 study was undertaken in response to concerns of crab 

 fishermen and resource managers that Grays Harbor 

 is important as a juvenile crab nursery. 



Dungeness crab provide major fisheries along the 

 west coast of North America, from central California 

 to southern Alaska (Botsford et al. 1989). Since 1945, 

 annual Washington coast crab landings have fluctuated 

 between 1.2 and 9.5 thousand metric tons per year (Fig. 

 2). The general life-history pattern of Dungeness crab 

 along the Washington coast is as follows (Gunderson 

 et al. 1990, Jamieson and Armstrong 1991). Females 

 molt to maturity along the open coast, generally in the 

 spring. Mating occurs at this time, but eggs are not 

 extruded until the following winter. Eggs generally 

 hatch between December and March, and larvae re- 

 main in the water column for a few months. Late-stage 

 larvae are found onshore in late-spring and summer, 

 where they settle to the bottom and metamorphose. 

 Settlement occurs both in nearshore coastal waters and 

 in estuaries; within estuaries, crab settle in both sub- 

 tidal and intertidal habitats. Crab settling in intertidal 



GRAYS HARBOR 

 WASHINGTON 



SCALE IN RiLOWETERS 



Figure 1 



Map of Grays Harbor, Washington, showing existing navigation channel (heavj' solid 

 line) and sampling strata (separated by dashed lines). 



areas may remain there during their first summer, but 

 move into the subtidal zone in fall. Few older crab are 

 resident in the intertidal, but move on and off the tidal 

 flats with the tides (Stevens et al. 1984). Crab settling 

 in nearshore waters may remain there for life, but 

 there is evidence of some migration into the estuary 

 between their first and second summers. Crab remain 

 in estuarine subtidal areas for up to two years, but late- 

 juvenile and early-adult crab leave the estuary before 

 reproduction, which occurs mainly along the open 

 coast. Both female and male crab reach sexual maturity 

 at about 2 years of age. but males may not breed until 

 age-3 or older (Butler 1960 and 1961, Hankin et al. 

 1989). 



Methods 



Model structure 



The calculation of crab loss is driven by two variables: 

 crab abundance (uncontrolled) and volume dredged 

 (controlled). Both of these vary in both space and time. 

 The two types of data are related through an entrain- 

 ment function that describes the number of crab en- 

 trained by each type of dredging gear as a function of 

 local crab density and volume dredged. Not all crab en- 

 trained are killed, so a second relationship describes 

 the number killed as a function of crab age and dredge 

 type. To apply the model, crab abundance is measured 



