FISHERY BULLETIN: VOL. 71, NO. 1 



detected. Some of the genera of helminths may 

 be represented, in fact, by more than one spe- 

 cies since specific identification of some imma- 

 ture forms was not possible. The parasites are 

 considered under five taxonomic groups: Mono- 

 genea, Digenea, Cestoda, Acanthocephala, and 

 Nematoda. Some data on the life cycle of each 

 parasite, which are essential to an understanding 

 of the cause-and-effect relationships for para- 

 sitic infections, are given where available. 



MONOGENEA 



No monogenetic trematodes of freshwater or- 

 igin were found on the gills of smolts, although 

 Tetraonchus alaskensis Price, 1937 has been re- 

 ported from young sockeye salmon in the Wood 

 River system (Margolis, 1963, tables). Gyro- 

 dactyloides strelkotvi (Gyrodactylidae) parasit- 

 ized the gills of five adults taken at Pederson 

 Point. This viviparid is specifically a salmon 

 parasite (Margolis, 1965) and is phylogeneti- 

 cally related to the genus Gyrodactylus which 

 parasitizes primarily freshwater teleosts. Since 

 G. strelkowi lives externally on its host and is 

 adapted only to marine conditions, it is lost soon 

 after the return of adult sockeye salmon to fresh 

 water. 



DIGENEA 



Two species of dignetic trematodes were ac- 

 quired by smolts in fresh water, and three spe- 

 cies by maturing fish in the sea. 



Metacercariae of the trematode genus Diplo- 

 stomulum (Diplostomatidae) occurred in the eye 

 lenses of over half of the smolts. The incidence 

 of infection was slightly less in age 1 (60%) than 

 in age 2 smolts (67%). Infection normally oc- 

 curs via furcocercous cercariae that emerge from 

 freshwater gastropods, penetrate the skin of 

 various fish, and localize in their eyes as metacer- 

 cariae. Studies on Diplostomum spathaceum 

 (Rud., 1819), a widely distributed Palearctic 

 species that matures in the intestine of gulls, 

 indicate that infection of fish does not occur in 

 brackish water, a result of the absence of re- 

 quired freshwater molluscan hosts (Cichowlas, 

 1961). The incidence of Diplostomulum in re- 



turning adults taken at Pederson Point was sig- 

 nificantly lower (15-16%) than in departing 

 smolts, but still less than in adults penetrating 

 into the headwaters of the Kvichak ; thus those 

 fish that returned to spawn were again exposed 

 to cercariae after re-entry. 



Variable numbers of three hemiurid trema- 

 todes were found in the stomach and upper in- 

 testine of adult sockeye salmon from Bristol Bay: 

 Lecithaster gibbosus, Branchyphallus crenatus, 

 and Tubulovesiciilalindbergi (Hemiuridae). All 

 were acquired in the marine environs by inges- 

 tion of crustacean plankters that serve as second 

 intermediate hosts and carry the stages maturing 

 in sockeye salmon. The incidence of L. gibbosus 

 in sockeye salmon at Pederson Point exceeded 

 90% and was only slightly less in adults col- 

 lected from the spawning grounds, Margolis 

 (1963) noted that the rate of infection by L. 

 gibbosus in sockeye salmon was lower in off'shore 

 than in inshore areas and deduced that most 

 parasites in maturing fish were acquired after 

 they re-entered coastal waters. This conclusion 

 was supported by data of Mamaev and Oshmarin 

 (1963) for sockeye salmon taken along the So- 

 viet eastern coast and of Boyce (1969) for those 

 from British Columbia waters. The acquisition 

 of the parasites in coastal areas is presumably 

 related to the relatively high abundance of mol- 

 luscan first intermediate hosts. Boyce (1969) 

 found that marine snails of the genus Thais were 

 the first intermediate host and the marine cope- 

 pods Centropages abdominalis and Pseudocakb- 

 nus minutus were the second intermediate hosts 

 of L. gibbosus in coastal areas of British Colum- 

 bia. L. gibbosus matures in 1 to 2 weeks and 

 has a lifespan of about 2 to 9 months in pink 

 salmon, whereas Tubulovesicula lindbergi ma- 

 tures in 2 to 4 months and lives at least 31 months 

 in chum salmon (Margolis and Boyce, 1969). 

 Thus most trematodes acquired in coastal waters 

 and maintained in the digestive tract of sockeye 

 salmon during their migration into the Kvichak 

 system are likely to persist until their post- 

 spawning death. 



Branchyphallus and Tubulovesicula were of 

 comparatively low incidence, and Hemiurus le- 

 vinseni Odhner, 1905, a marine hemiurid known 

 to infect adult salmon in the North Pacific, was 



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