LIVINGSTON: PACIFIC COD PREDATION ON THREE CRAB SPECIES 



portions of the estimated age 1 population and 

 could be an indication of overestimation of the 

 population consumption by cod, underestimation 

 of the age 1 population size because of unac- 

 counted sources of mortality or underestimation 

 of age 3 numbers by research trawl surveys. As 

 mentioned earlier, cod have been shown to con- 

 sume snow crab throughout the year in the east- 

 ern Bering Sea so the current estimate of 153 

 days for vulnerability to predation by cod is an 

 underestimate. Further, the cod diet contained 

 fair amounts of snow crab that could not be iden- 

 tified to the species level due to state of digestion 

 and were not included in the estimates. This 

 again leads to conservative estimates of the total 

 number of snow crab consumed by cod. 



A similar comparison of C. opilio removals at 

 age 1 by cod with trawl survey estimates of num- 

 bers remaining at age 3 shows that numbers 

 remaining at age 3 are greater than those eaten 

 at age 1 during 1981 and 1985. Percentages re- 

 moved of the reconstructed age 1 cohort would 

 be 28%, 57%, and 27% for 1981, 1984, and 1985, 

 respectively. These percentages are overesti- 

 mates because substantial numbers of age 2 C. 

 opilio are also eaten (Fig. 8) but not included in 

 the reconstructed population estimate. 



Although these estimates are subject to many 

 sources of error, the high predation mortality 

 rates of juvenile crab found here may not be 

 unrealistic. Using food habits data to quantify 

 predation removals, multispecies virtual popula- 

 tion analysis of North Sea fish stocks produced 

 average annual instantaneous predation mortal- 

 ity coefficients ranging from 0.2 to 1.8 for age 1 

 fish in the model (Daan 1987). Large interannual 

 differences in predation mortahty coefficients 

 were observed within fish species, suggesting 

 that predator populations were e.xerting den- 

 sity-dependent control on some year classes. 

 Similarly, our study has shown the possibility of 

 large predation removals of mostly age 1 snow 

 crabs and some interannual variation in the per- 

 centages removed. The impact on C. bairdi 

 seems greater than on C. opilio, at least for age 

 1 crab. Chionoecetes bairdi are also more vulner- 

 able to cod predation because of their high 

 spatial overlap with cod populations. In contrast, 

 unknown portions of the C. op/Zio juvenile popu- 

 lation are north of the main survey area, and the 

 main center of their distribution may shift south 

 into areas populated by cod in some years. 



Analysis of a longer time series of cod preda- 

 tion data may help locate abundant crab year 

 classes and allow us to track their numbers over 



time. There is uncertainty about the growth pat- 

 terns of juvenile snow crabs, and following size 

 class modes in cod stomach data in succeeding 

 years may provide more clues to these gi'owth 

 patterns. Further, although this study does not 

 attempt to e.xplain factors influencing early life 

 history survival, it does suggest that predation 

 is an important factor influencing survival of 

 ages 1 and 2 snow crab. 



ACKNOWLEDGMENTS 



I gratefully acknowledge the outstanding ef- 

 forts of every past and present member of the 

 Food Habits Progi-am, particularly those who 

 performed the laboratory analysis of cod stom- 

 achs: Mei-Sun Yang, Geoffrey Lang, Douglas 

 Milward, and Deborah Siefert. Special thanks 

 are in order for Allen Shimada and Jeffrey June 

 for the 1981 collections. I also thank all those 

 who contributed to or supported me in this re- 

 search. I am especially appreciative of the 

 thorough and thoughtful reviews given to this 

 paper, particularly by one critical reviewer. 

 Their efforts have helped to produce an im- 

 proved manuscript. 



LITERATURE CITED 



Adams, A. E. 



1979. The life history of the snow crab. Chionoecetes 

 opilio: A literature review. Alaska Sea Grant Rep. 

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 Bakkala, R. G., G. G. Thompson, and A. M. Shimada. 



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 Blau, S. F. 



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Conan, G. Y., and D. R. Maynard. 



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