and Rybock et al. (1975), we believe that popula- 

 tion differences do not explain the differences in 

 results between the studies of Rybock et al. (1975) 

 and Neilson et al. (1985). Furthermore, our study 

 provided strong evidence to support the conclu- 

 sion of Neilson et al. (1985) that the usefulness of 

 measurements of otolith nuclei to identify sym- 

 patric juvenile progeny of resident rainbow trout 

 and steelhead reared in the wild may be limited. 



Acknowledgments 



We thank Jeff Light for his advice on grinding 

 and polishing otoliths to resolve their nuclear di- 

 mensions and Eric Volk for his review of this 

 manuscript. This research was funded by 

 Bonneville Power Administration, U.S. Depart- 

 ment of Energy, Agreement No. DE-A179- 

 83BP13499. 



Literature Cited 



Fessler, J L. 



1972. An ecological and fish cultural study of summer 

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IHSSEN. P E . H E BooKE. J M Casselman, J M McGlade, N. R. 

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1981. Stock identification: materials and methods. Can. 

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McKern. J. L., H. F HoRTON, and K V. Koski 



1974. Development of steelhead trout (Salmo gairdneri) 

 otoliths and their use for age analysis and for separating 

 summer from winter races and wild from hatchery stocks. 

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Neilson. J D . G L Geen, and B Chan 



1985. Variability in dimensions of salmonid otolith nu- 

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Rybock. J. T 



1973. Use of otoliths to differentiate juvenile steelhead 

 trout from juvenile rainbow trout in the lower Deschutes 

 River, Oregon. M.S. Thesis, Oregon State University, 

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Rybock. J T , H F Horton, and J L Fessler. 



1975. Use of otoliths to separate juvenile steelhead trout 

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United States Department of the Interior Geological 

 Survey 



1967. Water resources data for Oregon, Pt. 2. U.S. Dep. 

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1968. Water resources data for Oregon, Pt. 2. U.S. Dep. 

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1969. Water resources data for Oregon, Pt. 2. U.S. Dep. 

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1982. Water resources data for Oregon water year 1982. 

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1983. Water resources data for Oregon water year 1983. 



U.S. Dep. Inter., Geol. Surv., Eastern Oregon, Vol. 1, 

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Kenneth P. Currens 



Carl B Schreck 



Hiram W. Li 



Oregon Cooperative Fishery Research Unit 

 Oregon State University 

 Corvallis. OR 973313 



3Cooperators are Oregon State University, Oregon Depart- 

 ment of Fish and Wildlife, and U.S. Fish and Wildlife Service. 



AGE-SPECIFIC VULNERABILITY OF 



PACIFIC SARDINE, SARDINOPS SAGAX, LARVAE 



TO PREDATION BY 



NORTHERN ANCHOVY, ENGRAULIS MORDAX 



To a large degree interannual variability in re- 

 cruitment determines the size of pelagic fish pop- 

 ulations. Recruitment to the Pacific sardine, 

 Sardinops sagax, population off California varies 

 from year to year over several orders of magni- 

 tude and is unrelated to spawning stock size 

 (Murphy 1966; MacCall 1979). Variable mortal- 

 ity rates in the first year of life must determine 

 year-class strength, although the sources of this 

 variability are unknown. Mortality rates in the 

 earliest stages are size specific with highest rates 

 in the egg and yolk-sac stage (Ahlstrom 1954; 

 Butler 1987) and may contribute to variability in 

 year-class strength (Smith 1985). 



The sources of mortality of sardine larvae have 

 yet to be investigated. In other pelagic larvae, 

 mortality is due to either starvation or predation, 

 and starvation is significant only during the brief 

 period after the onset of feeding (O'Connell 1980; 

 Hewitt et al. 1985; Theilacker 1986; Owen et al. 

 1987). In sardines, significant mortality occurs 

 during the egg and yolk-sac stages (Ahlstrom 

 1954) and this mortality can only be due to preda- 

 tion. Variable mortality in older larval and juve- 

 nile sardines may also contribute to variability in 

 recruitment, and this mortality, as in other 

 fishes, may also be due to predation (Hunter 

 1984). 



The objective of this paper was to determine the 

 size-specific vulnerability of Pacfiic sardine lar- 

 vae to predation by adult northern anchovies, 

 Engraulis mordax. The vulnerability of cape 

 anchovy and northern anchovy larvae to 



FISHERY BULLETIN: VOL. 86, NO. 1, 1988. 



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