Seasonal variations in the parasite infections in a population of three-spined sticklebacks,Gasterosteus aculeatusL.

A population of three-spined sticklebacks, Gasterosteus aculealus , from a pond in Somerset was sampled about every three weeks for 19 months, a total of 1863 fish being collected. Three species of parasites were found in the fish: Schistocephalus solidus , Diplostomum gasterostei and Echinorhynchus clavula. Seasonal changes in the incidence and intensity of the three parasitic infections are described. In general the infections increased in summer and autumn and decreased in winter and spring, although in Schistocephalus the differences in the level of infection between the two years were greater than between seasons. The largest increases in the Schistocephalus and Echinorhynchus infections occurred in August and September and in the Diplostomum infection in May and June. These increases could be related to the feeding activity of the sticklebacks and to the abundance and level of infection of the Cyclops , Asellus and Limnaea pereger populations. Reduction in the sizes of the infections was by predation and death of heavily infected fish in the case of Schistocephalus and Diplostomum and mainly by death of individual worms in Echinorhynchus. These mechanisms are shown to be adapted to the life cycles of the parasites. I am most grateful to Dr H. D. Crofton for his help and encouragement during this investigation and to Dr R. A. Avery for useful discussions and advice. This work was supported by a Science Research Council research studentship and a NATO studentship.