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



custa, Amphithoe rubricata, and Pontoporeia 

 femorata in European waters (Ginetsinskaya, 

 1958) and Cyphocaris challengeri off the coast 

 of British Columbia (Ekbaum, 1938). Infec- 

 tions are gradually lost because of the relatively 

 short lifespan of the mature parasite when an- 

 adromous salmon cease feeding (Ginetsinskaya, 

 1958). 



Postlarval forms of Bolbosoma sp. (Polymor- 

 phidae), probably B. caenoforme on ecological 

 grounds, were common in the intestine of adult 

 sockeye salmon from Pederson Point (60-74% 

 incidence), but the incidence decreased over a 

 3-month period as the fish penetrated into fresh 

 water. The decrease in incidence was accom- 

 panied by a decrease in mean intensity from 

 about 5 (range, 1-15) in fish taken from Bristol 

 Bay to about 2 (1-6) in fish collected on the 

 spawning grounds. Maturation of B. caenoforme 

 appears to be inhibited in salmon, which are 

 presumably accidental hosts. The definitive 

 hosts are believed to be marine mammals. Nev- 

 ertheless, there is a wide distribution and high 

 incidence of this acanthocephalan in salmon feed- 

 ing in waters of the North Pacific (Akhmerov, 

 1963; Mamaev et al., 1959; Margolis, 1963, 

 table) . 



Four species of juvenile Corynosoma (Poly- 

 morphidae) were reported in sockeye salmon 

 from the North Pacific and adjacent seas by 

 Margolis (1958) , but the incidence was less than 

 2%. One species, C. villosum Van Cleave, 1953, 

 was reported in Ugashik River sockeye salmon 

 in 1957 but not in other Bristol Bay adults (Mar- 

 golis, 1963, table) . No Corynosoma were found 

 in Kvichak adult sockeye salmon during this 

 study. 



NEMATODA 



Philonema oncorhyncki (Philometridae) was 

 the common nematode parasitizing the smolts, 

 but the incidence from our data was higher in 

 age 1 (100%) than in age 2 (92%). Larval 

 forms commonly occurred in the swim bladder, 

 occasionally in the kidney, and rarely in other 

 visceral organs. The larvae apparently leave 

 the swim bladder or tissues during the sockeye 

 salmon's sojourn at sea and enter the body ca- 



vity, where they mature in synchrony with ma- 

 turation of the host (Margolis, 1970). In the 

 adults taken at Pederson Point, the incidence 

 of P. oncorhynchi exceeded 70% , and the mature 

 worms were localized in the body cavity. Data 

 tabulated by Margolis (1963) indicate that the 

 incidence of this nematode in sockeye salmon re- 

 turning to major Bristol Bay tributaries is gen- 

 erally high (92 - lOO^'r) and that the intensity 

 of the infections is relatively high (averaging 

 11-61 parasites/fish). There is little doubt that 

 the very small nematodes reported in sockeye 

 salmon smolts leaving Bristol Bay tributaries 

 in 1956 (100% incidence) by Margolis (1963) 

 were, as he assumed, larvae of P. oncorhynchi. 



Recent data have elucidated the life cycle of 

 this dracunculoid nematode, which infects salm- 

 on ids in fresh water on both borders of the North 

 Pacific Ocean. Gravid worms are presumably 

 discharged along with eggs when the adult fish 

 spawn, the worms burst to liberate thousands of 

 larvae, and the larvae are ingested by a suitable 

 copepod intermediate host, such as Cyclops bi- 

 cuspidatus (Platzer and Adams, 1967). Invasive 

 larvae are liberated from infected copepods in 

 the stomach of sockeye smolts, penetrate the 

 gut wall, and move through either the coelomic 

 cavity or the mesentery and associated tissues 

 to the swim bladder within 18 hr (Adams, 1969) . 

 Numbers of larvae carried by smolts moving sea- 

 ward may reach the hundreds (Margolis, 1970), 

 since the intensity of infection tends to increase 

 during the freshwater life of the planktivorous 

 smolts. 



Two other nematodes were acquired by the 

 smolts, Rhabdochona sp. and Contracaecum sp. 

 Rhabdochona (Rhabdochonidae) occurred in the 

 intestine of a few age 2 smolts (5%) but not 

 in adult sockeye salmon taken at Pederson Point. 

 Although little is known of the bionomics of var- 

 ious species of Rhabdochona, these nematodes 

 are common in various North American fresh- 

 water fishes (Choquette, 1951). The incidence 

 of Contracaecum was also low in age 1 (2%) 

 and age 2 (12% ) smolts, and about one immature 

 nematode was found encysted in the viscera of 

 each infected fish. 



Contracaecum and Anisakis (Heterocheilidae) 

 are cosmopolitan parasites of many marine fish- 



273 



