The occurence of plerocercoids of Schistocephalus solidus in the Fraser Valley and their effect on the intermediate host Gasterosteus aculeatus.

Samples of Gasterosteus aculeatus from 16 areas in the Fraser Valley and environs were examined for plerocercoids of Schistocephalus solidus. Fish at Coal Harbour and Alouette Lake were sampled several times over a twelve month period. The number and sizes of worms present in the Alouette Lake fish samples were recorded, and it was found that infected fish less than 45 mm. total length carried on average more worms than those over 45 mm., and that uninfected adult fish were caught only during the breeding season. In another lake, infected fish were found in a different area from the uninfected ones. The fish intermediate host was shown to be affected by the infection in four ways: (i) Infected fish died sooner than uninfected fish, (ii) Heavily infected fish were lighter in weight of fish tissue than controls of the same length, (iii) The total standard respiration rate of infected fish was higher than that calculated by combining values obtained from uninfected fish and published values for in vitro plerocercoids. (iv) Heavily infected fish required up to twice as much oxygen per gram fish weight per hour when swimming at the same speed as control fish. Other aspects were examined but the results were inconclusive or negative. The observations on natural populations are discussed in the light of the experimental findings.Science, Faculty ofZoology, Department ofGraduat