WASHINGTON 



"T Bottom trawl station 

 * Intertidal station 



PACIFIC 

 OCEAN 



1 ? 3 Miles 



I 1 I I 



I — ! — 1 — I 1 1 



12 3 4 5 Kilometers 



Figure 1.— Map of the Columbia River estuary and adjacent coastal areas, showing sampling sites for the 2-yr Dungeness crab study. 



Results and Discussion 



The prevalence of C. errans on Dungeness crabs 

 collected in the estuary was significantly lower than 

 the prevalence on crabs in the ocean (x^, df = 1, P 



< 0.001); average prevalences in the estuary and 

 ocean were 6 and 79%, respectively (Tables 1, 2). 

 Within the estuary, mean prevalence was highest 

 at the mouth (stations 1,2, 23-26) where it averaged 

 25%. In the estuary, significantly more females (8%) 

 were infested than were males (5%) (x", df = 1, P 



< 0.001), but in the ocean there was no significant 

 difference (P > 0.05) in prevalence on males (80%) 

 and females (76%). Only three egg-bearing females 

 were collected during the study; they were collected 

 December 1984 at the mouth of the estuary and in 

 the ocean. One egg-bearing female had an obvious 

 C. errans infestation. 



In both the estuary and ocean, size class I Dunge- 

 ness crabs were least frequently infested. No chi- 

 square comparison was done for this size class 

 because of the small numbers of infested crabs. In 

 addition, the total sample size of size class I crabs 

 in the ocean was small (46 crabs). For the individual 

 size classes II-IV, the prevalences of C. errans on 

 crabs were significantly lower in the estuary than 

 in the ocean (x^, df = 1, P < 0.001). In the estuary, 

 the infestation by C. errans was highest in size class 

 IV crabs (29%). 



The prevalence of C. errans found on Dungeness 

 crabs in the ocean and the Columbia River estuary 



was lower than the prevalence reported by Wickham 

 (1980) in the Bodega Bay, CA, area; he reported that 

 all nonegg-bearing crabs >20 mm carapace width 

 were infested with C. errans. In our study, some 

 light infestations may have been missed by not ex- 

 amining the entire exoskeletons of the Dungeness 

 crabs. 



The major result of our examinations for C. errans 

 was discovering the large difference in infestation 

 levels between the ocean (79%) and the estuary (6%). 

 Low salinities in the estuary, particularly upstream 

 from the mouth, probably were the major cause of 

 the lower infestation. During low river flows (about 

 4,400 m^/s), when salinity intrusion is greatest, 

 minimum bottom salinities in most of the lower 22 

 km of the estuary generally range from 0.5 to 15 

 ppt, although maximum salinities are >S0 ppt. Dur- 

 ing high river flows (about 8,800 m^/s), minimum 

 bottom salinities in much of the lower 22 km of the 

 estuary may be zero (Jay 1984). Wickham^ noted 

 that "Pure fresh water will kill worms in 1-2 minutes 

 depending on the worms' size." The lower 

 prevalence in the estuary may have have little ef- 

 fect on the overall prevalence in the ocean. Non- 

 infested Dungeness crabs migrating from the 

 estuary could be infested in the ocean by larval 

 worms (Wickham 1980), or through copulation 

 (Wickham et al. 1984). 



'D. E. Wickham, Bodega Marine Laboratory, P.O. Box 247, 

 Bodega Bay, CA 94923, pers. commun. November 1985. 



141 



