ington coast, C. magister females generally molt 

 and breed in April through June (Cleaver 1949), 

 and local residents are accustomed to the presence 

 of numerous cast shells on the beach at that time. 

 Since it occurred in the spring, the mortality, prior 

 to our investigation, was probably mistaken for 

 normal molting by many people including patrol- 

 men of the local state parks. It is possible, there- 

 fore, that similar occurrences have gone unre- 

 ported in the past due to such confusion; careful 

 examination of the shells is required to detect tis- 

 sues and distinguish dead animals from exuvia. 



Since the original drafting of this report, two 

 additional mortalities have come to our attention. 

 On 29 November 1979, Walt Cooke^ observed a 

 large number of dead female Dungeness crabs on 

 the beach from Ocean Park to Long Beach, Wash. 

 (15.6 km). He estimated their number to be 955- 

 1,910, ofwhich only 2. S'/f were exuviae, and 939^ of 

 the remainder were females averaging 146 mm ( n 

 = 58) carapace width. On 30 January 1980, Dar- 

 rell Demory estimated a mortality of 68 crabs/km 

 from the Umpqua River mouth north to 

 Tahkenitch Creek, Oreg., (13.9 km) for an esti- 

 mated total of 947, of which approximately 95% 

 were females. Conditions of both these occur- 

 rences were similar to the April mortality; each 

 consisted almost entirely of large female crabs 

 confined to the high tide line, and occasionally in 

 patches. 



It is noteworthy that several instances of dead 

 Dungeness crabs in commercial pots from Willapa 

 Bay prompted investigations by one of the au- 

 thors, David Armstrong, and personnel from WDF 

 and the National Marine Fisheries Service in Feb- 

 ruary and March 1979 (Tufts"*). Moribund and 

 dead crabs (male and female) were found in pots 

 checked every 24 h, and some were recovered from 

 shallow intertidal areas. This indicates that ani- 

 mals attracted to bait were healthy enough to 

 enter pots, and then died within a short time 

 thereafter. These observations were made follow- 

 ing a severe cold spell during which the air tem- 

 perature was below freezing for 2 wk, and parts of 

 Willapa Bay had frozen over suggesting tempera- 

 ture stress associated with the mortalities. 



Causes of the mortality of April 1979 are only 

 speculative, since crabs could not be examined his- 



tologically. Immediately after verifying the re- 

 port, we questioned WDF personnel to learn if 

 nearshore groundfisheries might account for ex- 

 tensive crab mortality by trawling activity, and 

 learned that no trawlers were known to be operat- 

 ing just prior to 18 April offshore from the transect 

 locations. We surmise that death was due to dis- 

 ease, and the specificity shown for large, older 

 females indicates a causative agent linked 

 perhaps to a decline in vigor with old age. Female 

 C. magister reach sexual maturity at about 100 

 mm carapace width in their second year and rela- 

 tively few animals exceed 145-150 mm (Butler 

 1961; Poole 1967). The average carapace width of 

 147 mm for dead crabs at transect 3 shows a pre- 

 ponderance of older females were affected, 

 perhaps because they were more susceptible to 

 possible lethal stresses than younger animals. 



Tissue samples dissected from moribund ani- 

 mals in Willapa Bay in March 1979 were studied 

 for histopathological anomalies, and preliminary 

 results indicate a systemic bacterial infection as a 

 possible causative agent according to Sparks. 

 Lethal diseases among wild populations of C. 

 magister are unknown but mortalities of 

 laboratory-held animals have been attributed to 

 such pathogens as fungus and ciliates (Armstrong 

 et al. 1976; Armstrong and Sparks ). It seems 

 likely that bacteria or various pathogenic proto- 

 zoans may take a toll in wild populations, particu- 

 larly in a segment of the population weakened or 

 predisposed by contributing factors such as old 

 age. 



Demory (footnote 3) concluded that the mortal- 

 ity observed in January 1980 on the Oregon 

 beaches may have been the result of stress on crabs 

 trapped in pots that were "sanded in" during a 

 period of adverse weather immediately preceding 

 the observation, when fishermen could not pull 

 these pots for several days. No diseases or para- 

 sites were found in specimens obtained from this 

 mortality. 



Although examples of mass mortality in crusta- 

 cean populations are rare, the presence of numer- 

 ous dead C. magister along a beach is reminiscent 

 of the "Gray Crab Disease" affecting Callinectes 

 sapidus on the east coast of the United States. 

 Lethal infections attributed to the amoeba 



^Walt Cook, Washington Department of Fisheries, PO. Box 

 158, Ocean Park, WA 98640, pers. commun. 2 June 1980. 



'Darrell Demory, Oregon State Department of Fish and 

 Wildlife, Newport, Oreg., pers. commun. 5 February 1980. 



^Dennis Tufts, Washington Department of Fisheries, P.O. Box 

 158, Ocean Park, WA 98640, pers. commun. 11 July 1980. 



^Albert Sparks, National Marine Fisheries Service, NOAA, 

 2725 Montlake Blvd. E.. Seattle, Wash., pers. commun. March 

 1979. 



"D. Armstrong and A. Sparks, College of Fisheries, University 

 of Washington, Seattle, WA 98195, unpubl. data on blood ciliates. 



351 



