Diphyllobothrium (Cestoda) Infections in Salmonids from Three Washington Lakes

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Abstract
The dynamics of infection by larval Diphyllobothrium (Cestoda) in populations of salmonid fishes from 3 western Washington lakes (Goodwin, Shoecraft, and Silver) were studied from June, 1963, to June, 1966. Plerocercoids were acquired by the fish primarily during the summer when subsurface water temperatures (0-20 ft) were at an annual maximum. Intensities of infection were generally higher in rainbow trout (Salmo gairdneri) from Goodwin Lake than in rainbows from Shoecraft and Silver lakes in each season. Comparison of infections in rainbows with that in other species of salmonids in Goodwin Lake revealad distinctly lower intensities in lacustrine coho salmon (Oncorhynchus kisutch). but only slightly lower intensities (one season of observations) in cutthroat (Salmo clarki). and brook (Salvelinus fontinalis) trout. These differences were probably relative to feeding rates of each species on zooplankton. Chronic mortalities occurred in association with warm temperatures during the initial spring, summer, and fall after the fish were planted, when plerocercoid development was comparatively rapid and the fish were relatively small. An epizootic, however, occurred in Goodwin Lake in the winter of 1963-64, when infections reached an average of 78 plerocercoids/fish in spring-plant rainbows, 140 in fall-plant rainbows, and 38 in spring-plant cohos. The factors responsible for this outbreak remain unknown. Considerations for the control of infections in salmonids based on the parasite''s life cycle and host relationships are discussed.

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