and 1975 were years with an unusually weak Califor- 

 nia Countercurrent, or conversely, stronger than nor- 

 mal southward flow and cooler than normal coastal 

 waters. If yearling coho do move directly into Califor- 

 nia waters after emigration from freshwater, then 

 these anomalous conditions may have caused these 

 fish to move farther south than usual, with the result 

 that adult coho would have been south of the preda- 

 tion zone critical to the northern California Dunge- 

 ness crab population in the spring of 1974 and 1976. 



Since 1976 O.P.I, area coho landings have under- 

 gone an inexplicable decline in spite of increasing 

 hatchery production. Theoretically, an increase in 

 Dungeness crab survival should have accompanied 

 this drop in coho survival. The drop in Dungeness 

 crab survival, evident in Figure 5, is obviously in- 

 consistent with the general theory but can be ex- 

 plained in two ways. First, it should be considered 

 that the earlier O.P.I, area coho landings contained 

 far fewer hatchery fish than those during the later 

 years. It has been estimated that hatchery fish com- 

 prised 75% of the west coast coho catch by 1977 

 (Scarnecchia and Wagner 1980). Secondly, a coastal 

 warming trend that began in 1976 (McLain 1983) 

 may have resulted in a northward shift in coho 

 distribution with a concomitant reduction in Dunge- 

 ness crab megalops survival. 



If coho have become the major limiter of Dunge- 

 ness crab megalops survival within California, then 

 the observed survival patterns suggest that a group 

 of coho, possibly representing the original hatchery 

 stocks, still experience consistently good survival 

 and continue to move into the predation zone critical 

 to the central California population. On the other 

 hand, the more irregular Dungeness crab survival 

 observed in northern California suggest that 

 megalops survival there is more dependent on the 

 vagaries of hatchery-reared coho distribution 

 associated with environmental nuances. 



Admittedly, most of the evidence used to support 

 the predator-prey hypothesis is circumstantial. 

 Nevertheless, three of the considerations presented 

 —1) the fact that coho feed heavily on Dungeness 

 crab megalops, 2) the evidence showing that many, 

 if not most, Oregon and Washington hatchery coho 

 are in California during the period megalops are 

 most abundant, and 3) the coincidence of the ex- 

 tended central California Dungeness crab decline 

 with a large increase in the number of hatchery coho 

 within the O.P.I, area— suggest a possible relation- 

 ship that deserves attention. 



The capricious nature of predation on the early life 

 stages of commercially important invertebrates un- 

 doubtedly contributes to the difficulties encountered 



when attempting to manage these relatively short- 

 lived species on a sustained yield basis. If the hypo- 

 thesized relationship between coho salmon and 

 Dungeness crab eventually proves to be correct, then 

 the salmonid enhancement process itself can be con- 

 sidered an experiment, offering insight into the role 

 predators play in controlling the commercial abun- 

 dance of many marine species. 



Acknowledgments 



I would like to thank Bob Tksto, John Geibel, Ron 

 Warner, and two anonymous reviewers for review- 

 ing the manuscript. Special thanks are also due Bob 

 Tksto, who offered considerable encouragement, and 

 Anita Thomas for drafting the figures. 



Literature Cited 



Allen, G. 



1965. Estimating error associated with ocean recoveries of 

 fin-marked coho salmon. Trans. Am. Fish. Soc 94:319-326. 

 BOTSFORD, L. W. 



1984. Effect of individual growth rates on expected behavior 

 of the northern California Dungeness crab {Cancer magister) 

 fishery. Can. J. Fish. Aquat. Sci. 41:99-107. 

 Eggers, D. M. 



1976. Theoretical effect of schooling by planktivorous fish 

 predators on rate of prey consumption. J. Can. Fish. Res. 

 Board 33:1964-1971. 

 Fry, D. H., Jr. 



1973. Anadromous fishes of California. Calif. Dep. Fish 

 Game, Resourc Agency, 111 p. 

 Glova, G. J. 



1978. Behavioral differences between wild and hatchery- 

 produced coho salmon juveniles and their management im- 

 plications. In B. G. Shepherd and R. M. J. Ginetz (Rap- 

 porteurs), Proceedings of the 1977 Northeast Pacific Chinook 

 and Coho Salmon Workshop, p. 84-88. Can. Fish. Mar. Serv, 

 Tfech. Rep. 759. 

 Godfrey, H., K. A. Henry, and S. Machidorl 



1975. Distribution and abundance of coho salmon in offshore 

 waters of the North Pacific Ocean. Int. North Pac Fish. 

 Comm., Bull. 31:1-80. 

 Hartt, a. C. 



1980. Juvenile salmonids in the oceanic ecosystem— the 

 critical first summer. In W. J. McNeil and D. C. Himsworth 

 (editors), Salmonid ecosystems of the North Pacific, p. 

 25-27. Oreg. State Univ. Press. 

 Hatfield, S. E. 



1983. Distribution of zooplankton in association with Dunge- 

 ness crab, Cancer magister, larvae in California. In P. W. 

 Wild and R. N. Tksto (editors). Life history, environment, and 

 mariculture studies of the Dungeness crab. Cancer magister, 

 with emphasis on the central California fishery resource, p. 

 97-123. Calif. Dep. Fish Game, Fish Bull. 172. 

 Heg, R., and J. Van Hyning. 



1951. Food of the chinook and silver salmon taken off the 

 Oregon coast. Oreg. Fish. Comm., Res. Briefs 3(2):32-40. 

 HOLLING, C. S. 



1959. The components of predation as revealed by a study of 



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