PENNELL, BECKER, and SCOFIELD: HELMINTHS OF KVICHAK SOCKEYE SALMON 



Table 2. — Incidence and intensity of some helminths from juvenile sockeye salmon in the Kvichak River system 

 in 1961, 1966, 1968, and 1969. (Incidence is the percentage of the sample infected; mean intensity, from infected 

 fish only, in parentheses.) 



^ * — larval parasites (in fish intermediate host). 



2 Examinations other than in 1969 were taken from fish preserved in 10% Formalin, which restricted accurate counts of Diplostomulum and P. oncorhynchi. 



study (Table 2). Data from Kvichak smolts in 

 1964 and 1965 (Margolis, 1967) suggest that 

 the incidence of T. crassus in Kvichak smolts 

 is generally low, less than 3%, and that the 

 usual intensity is but one or sometimes two ple- 

 rocercoids per parasitized fish. T. crassus has 

 been found regularly in samples from the Wood 

 and Naknek Rivers but only occasionally in those 

 from the Ugashik and Kvichak Rivers (Margolis, 

 1967). 



Metacercariae of Diplostomulum occurred in 

 the eye lenses of 60% of the Kvichak smolts in 

 this study but the mean intensities were low, 2.0 

 to 2.5 parasites per fish. An incidence of 48% 

 (25 fish examined) and an intensity of 2.0 in 

 Kvichak smolts studied in 1956 were tabulated 

 by Margolis (1963). 



Margolis (1963) found that 15% of the Kvi- 

 chak adults returning in 1957 were infected with 

 Diplostomulum (mean intensity, 2) and our data 

 (Table 1) closely agree. Thus the incidence but 

 not the intensity of the infections is significantly 

 lower in returning adults than in smolts. Wheth- 

 er this is due to the loss of metacercariae in 

 the lenses of many ocean-dwelling fish, as sug- 

 gested by Margolis and by Dogiel (1966) , or per- 

 haps to disproportionate loss of infected fish as 

 our data hint is conjectural. 



The incidence of DiphyllobothriuTn in age 2 

 Kvichak smolts was 66% from data obtained dur- 

 ing this study, but intensities were usually low, 

 with one or two plerocercoids per infected fish. 

 Of the smolts examined from Bristol Bay tribu- 

 taries in 1957 by Margolis (1963), Kvichak fish 

 ranked second in level of incidence (52%), 

 whereas those from the Egegik River were first 



with 88% incidence, and those from the Naknek 

 were third with 20% incidence. 



Data obtained in this study and provided by 

 Margolis (1963) indicate that nearly all smolts 

 leaving the major rivers of Bristol Bay are in- 

 fected with larvae of Philonema oncorhynchi, 

 but that the intensities vary widely, often ex- 

 ceeding 50 per fish. 



It is beyond the scope of this paper to pos- 

 tulate on any eflfects that the parasites of Kvichak 

 sockeye salmon may have upon the survival of 

 their host. There is ample evidence in the lit- 

 erature that, given certain pre-disposing con- 

 ditions, direct mortalities from mechanical dam- 

 age, histopathological changes, and measurable 

 eflfects upon size and growth rates of parasitized 

 fish do occur. Any factor that results in environ- 

 mental discrimination, such as the presence of 

 internal and external parasites, would tend to 

 lessen the chances of a fish for survival. The 

 question of whether or not the various parasites 

 occurring in Kvichak sockeye salmon, alone or 

 in combination, cause losses of ecological signifi- 

 cance remains to be explored. 



POSTSCRIPT 



David A. Pennell entered the College of Fish- 

 eries of the University of Washington as a grad- 

 uate student in the fall of 1968. On August 1969, 

 he was involved in a fatal airplane accident while 

 collecting material in the field for his M.S. dis- 

 sertation on the parasites of sockeye salmon in 

 Bristol Bay, Alaska. This paper is published 

 posthumously in honor of this promising and 

 highly dedicated young man. 



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