HELMINTHS OF SOCKEYE SALMON {ONCORHYNCHUS NERKA) 

 FROM THE KVICHAK RIVER SYSTEM, BRISTOL BAY, ALASKA' 



David A. Pennell,' C. Dale Becker," and Nora R. Scofield* 



ABSTRACT 



A study of helminths infecting juvenile and adult sockeye salmon (Oncorhynchus nerka) 

 leaving and entering the Kvichak River system, Bristol Bay, Alaska, was conducted in 

 1969. Ten helminths acquired in fresh water were found in smolts: Diplostomulum sp. ; 

 an unidentified trematode; Diphyllobothrium spp. ; Triaenophorus crassus Forel, 1868: 

 Proteocephalus sp. ; Eubothrium salvelini (Schrank, 1790); Neoechinorhynchus rutili 

 (Mueller, 1789) ; Philonema oncorhynchi Kuitunen-Ekbaum, 1933; Rhabdochona sp.; 

 and Contracaeciim sp. In addition to surviving larval stages of freshwater parasites, 

 adults were infected by nine helminths acquired in the sea: Gyrodactyloides strelkowi 

 Bykhovskaya and Polyanskaya, 1953; Lecithaster gibbosus (Rud., 1802) ; Brachyphallus 

 crenatus (Rud., 1802) ; Tubulovesicula lindbergi (Layman, 1930) ; Phyllobothrium. 

 caudatum (Zschokke and Heitz, 1914) ; Echinorhynchus gadi Mueller, 1776; Bolbosoma 

 caenoforme Heitz, 1920; Anisakis sp. ; and Contracaecum sp. Infection incidences and 

 intensities are tabulated where accurate data are available. Information on life histories 

 is assembled from scattered sources, and some ecological aspects of helminths infecting 

 Kvichak sockeye salmon are briefly discussed. 



The Kvichak River system is the largest pro- 

 ducer of sockeye salmon, Oncorhynchus nerka 

 (Walbaum), among five river systems in Bristol 

 Bay, western Alaska. Its drainage basin covers 

 nearly 8,000 square miles and includes two large 

 lakes, Iliamna Lake (90 miles long and up to 

 26 miles wide) and Lake Clark (50 miles long 

 and up to 4 miles wide). Intensive studies on 

 sockeye salmon from this system and the other 

 four systems, the Wood River, Naknek River, 

 Egegik River, and Ugashik River, have been 

 underway since 1946 (Thompson, 1962) . Recent 

 research has been directed toward determining 

 the biological basis for annual fluctuations in 



^ This study was supported by funds provided by the 

 Washington Sea Grant Program, which is maintained 

 by the National Oceanic and Atmospheric Administra- 

 tion of the U.S. Department of Commerce. Contribution 

 No. 369, College of Fisheries, University of Washington. 



' Fisheries Research Institute, College of Fisheries, 

 University of Washington, Seattle, WA 98105; now de- 

 ceased. 



^ Ecosystems Department, Battelle Memorial Insti- 

 tute, Pacific Northwest Laboratories, Richland, WA 

 99352. 



* Fisheries Research Institute, College of Fisheries, 

 University of Washington, Seattle, WA 98105; present 

 address: University of Idaho, Moscow, Idaho 83843. 



Manuscript accepted January 1972. 



FISHERY BULLETIN: VOL. 71, NO. 1, 1973. 



the number of seaward migrants and subsequent 

 return of adults 2 and 3 years later, and pro- 

 viding a reliable estimate of the optimum escape- 

 ment for each of these river systems (Burgner 

 et al., 1969). 



The survival rate for Kvichak River sockeye 

 salmon is generally higher in the peak year of 

 the abundance cycle than in other years since 

 the total production from each year class comes 

 in installments 4, 5, or 6 years later, depending 

 on the total age of the adults when they return 

 to spawn. The occurrence of mortality inversely 

 related to density has been ascertained from ob- 

 servations, but its causes remain unidentified. 

 Since the available records show that, generally, 

 smolt production has been proportionate to 

 adult escapement, it appears that losses occur 

 primarily after the smolts leave the nursery 

 areas, but the possibility remains that the pre- 

 disposing conditions are found in fresh water. 

 The adjustments that smolts make to environ- 

 mental changes, their physiological condition, 

 and their resistance to stress have been studied 

 by those who seek the factors that underlie var- 

 iations in survival rate among year classes in 



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