FISHERY BULLETIN: VOL. 82, NO. 3 



trawl, the total number of trawl-catchable crabs 

 present in 1980-81, ±95% confidence intervals, 

 were summer, 4.3 ± 1.7 million crabs; winter, 1.3 ± 

 0.7 million crabs; and spring, 2.6 ± 1.2 million 

 crabs (Table 5). However, stratum 1 (837 ha) was 

 excluded from the spring estimate due to lack of 

 data, but was reincluded later, using density esti- 

 mates from adjacent stratum 3. Totals for the 

 other nonsampled strata (10, 11, 14, and 15) were 

 added to totals for the trawl-sampled strata, and 

 the sums for each age group were divided by the 

 trawl efficiency estimates described above. Final 

 calculated numbers for the total crab population 

 were summer, 39.0 million crabs; fall-winter, 3.3 

 million crabs; spring, 7.8 million crabs (Table 5). 



The 1980 year class, which was extremely abun- 

 dant in the summer of 1980, virtually disappeared 

 during the following winter, and reappeared in 

 spring of 1981. Some hypotheses for this decline 

 and recovery include winter hibernation, migra- 

 tion to nonsampled areas (e.g., stratum 14), and 

 temporary egress from the estuary. Natural mor- 

 tality probably contributed substantially to the 

 decline as well. 



Due to the speculative nature of these estimates 

 and the underlying assumptions concerning trawl 

 efficiency and proportion of habitat utilized, it was 

 not possible to compute confidence limits on these 

 final estimates. The estimates of total population 

 abundance in the estuary suggest a tremendous 

 increase in summer with the influx of 0+ crabs as 

 megalopae and first instars, and an increase in 1 + 



animals as well. This estimate of 39 million crabs 

 is the highest estuarine crab population abun- 

 dance yet reported. The only other reported esti- 

 mate, that of 9.3 million crabs in the San 

 Francisco-San Pablo estuary complex during 1975 

 (Orcutt 1978), is based on a much less systematic 

 survey than ours and does not correct for poor gear 

 efficiency in regards to the small size of early in- 

 stars. Furthermore, this latter estuarine system 

 represents an area (500 km 2 ) five times that of 

 Grays Harbor. 



The accuracy of our estimates of population 

 abundance can be qualitatively assessed by com- 

 parison of trawl density data with that of other 

 studies (Gotshall 1978a; Orcutt et al. 1975, 1976; 

 Orcutt 1977, 1978; Table 6). Generally, there is 

 great seasonal variation, but densities estimated 

 in Grays Harbor are in accord with values for 

 Humboldt Bay and San Francisco Bay. Extrapola- 

 tions to total abundance indicate that large popu- 

 lations of juvenile crabs may use other coastal 

 estuaries as well. Even relatively small estuaries 

 in Oregon, such as Tillamook, Netarts, Yaquina, 

 and Coos Bay, could support large populations of 

 0+ crabs, considering their small biomass (0.2 g 

 dry weight). The principal benefits of these es- 

 tuaries are probably refuge from larger can- 

 nibalistic conspecifics (Botsford and Wickham 

 1978; Stevens et al. 1982), more abundant food, and 

 possibly accelerated growth as a result of food 

 supplies and warmer temperatures than offshore 

 waters. 



TABLE 5. — Estimation of Cancer magister population in Grays Harbor, Wash., for 

 1980-81. All values are numbers of crabs except efficiency factors and percentages. 

 C.I. = confidence interval. 



'See Table 3 and Figure 7 for size of these age classes throughout the year 1980-81 



2 Values for t(0.05)- summer = 2.029, winter = 2.048, spring = 2.160. 



3 See text for explanation of estimates based on data of adjacent trawl stations 



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