DISTRIBUTION, ABUNDANCE, AND GROWTH OF 



JUVENILE DUNGENESS CRABS, CANCER MAGISTER, IN 



GRAYS HARBOR ESTUARY, WASHINGTON 1 



Bradley G. Stevens 2 and David A. Armstrong 3 



ABSTRACT 



Dungeness crabs, Cancer magister, were collected biweekly or monthly from May 1980 to July 1981 in 

 Grays Harbor, Washington. Age of each crab was estimated from width-frequency analyses, and the 

 population density and growth rate were monitored for each age class over the 14-month period. In 

 April 1980 and 1981, crabs entered the estuary either as megalops larvae that metamorphosed to first 

 instar postlarvae or directly as first instars. Intertidal mudflats with beds of eelgrass (Zostera spp.) 

 were important habitats for the first few postlarval stages. Some crabs may have emigrated from the 

 estuary during their second year of life, whereas others dispersed throughout the estuary and appeared 

 to emigrate at sexual maturity (about 2 years). No gravid females were ever found in the bay. 

 Population size was estimated to range from 3.3 million crabs (winter) to 39.0 million crabs (summer); 

 74% of the summer population were early instars. Growth of early instars was rapid and resulted in a 

 282-fold increase in dry weight from May to September, but little growth occurred during the remain- 

 der of the year. Based on summer population abundance, it is estimated that this estuary could account 

 for a substantial portion of recruitment to the offshore commercial fisheries. 



The biology of the Dungeness crab, Cancer magis- 

 ter, has been studied by numerous investigators 

 for several decades (Weymouth and MacKay 1936; 

 MacKay 1942) because of its importance to com- 

 mercial fisheries and its position as a benthic 

 predator in estuaries and offshore communities 

 (Gotshall 1977; California Department of Fish and 

 Game 1981). Previous studies of C. magister biol- 

 ogy have been conducted largely along the open 

 coast (MacKay 1942; Cleaver 1949; Butler 1960, 

 1961; Gotshall 1978b, c). The few studies of crab 

 populations in estuaries or shallow- water habitats 

 (Butler 1956; Tegelberg and Arthur 1977; Gotshall 

 1978a; California Department of Fish and Game 

 1981) have indicated that such areas may be ex- 

 tremely important nursery grounds, but the size 

 and dynamics of estuarine populations have not 

 been statistically determined and, furthermore, 

 the contribution of estuarine habitats to offshore 

 stocks has not been adequately assessed. Orcutt et 

 al. (1978) estimated that 50-80% of crabs caught 

 by the fishery in the Gulf of the Farallones spend 

 some of their life cycle in the San Francisco-San 



'Contribution No. 629, School of Fisheries, University of 

 Washington, Seattle, Wash. 



2 School of Fisheries, WH-10, University of Washington, 

 Seattle, Wash.; present address: National Marine Fisheries 

 Service, P.O. Box 1638, Kodiak, AK 99615. 



3 School of Fisheries, WH-10, University of Washington, 

 Seattle, WA 98195. 



Manuscript accepted January 1984. 

 FISHERY BULLETIN: VOL. 82, NO. 3, 1984. 



Pablo Bay complex. Benefits derived from es- 

 tuarine early life history may include enhanced 

 growth rates, more abundant food, and refuge for 

 postlarval and juvenile crabs from larger, older 

 age classes that act as competitors and predators 

 (Botsford and Wickham 1978). 



Quantitative studies of C. magister in major 

 estuaries are timely and imperative. The demise of 

 the San Francisco fishery prompted a 5-yr investi- 

 gation of C. magister biology in that region 

 (California Department of Fish and Game 1981), 

 and hypotheses for the decline include alterations 

 of estuarine habitat and water quality. In addition, 

 channel dredging practices in west coast estuaries 

 kill hundreds of thousands of crabs annually (Ste- 

 vens 1981). Armstrong et al. (1982) estimated that 

 a proposed channel modification project in Grays 

 Harbor, Wash., could entrain and kill 2.5 million 

 crabs over a 2-yr period. Knowledge of estuarine 

 crab population dynamics and ecology of juveniles 

 is required to gauge the relative importance of 

 such habitat to the species, and to mitigate im- 

 pacts of estuarine development on juvenile stages. 



MATERIALS AND METHODS 

 Study site 



Grays Harbor is a shallow drowned river basin 



469 

 t/#3 



