Thersitina gasterostei (Pagenstecher, 1861) (Copepoda: Ergasilidae) infecting the stickleback Pungitius pungitius L. at Chalk Marshes, Gravesend, Kent.

10-spined sticklebacks, Pungitius pungitius, taken from a drainage dyke at Chalk Marshes, Kent, between October 1979 and July 1980 were infested with adult females of the parasitic copepod Thersitina gasterostei. Relative density of the overwintering parasite population fell during the winter months to a minimum in April; however during this winter period the proportion of ovigerous females rose, reaching 100% in April. Young infective adults were first seen in May, and the peak of infection by the first summer generation occurred in June. The parasite was found to be overdispersed within the host population: in 8 of 10 monthly samples the negative binomial distribution was a good model for the observed data. Relative density was related to host length in only 3 of the 10 samples. Evidence is presented to suggest that infected fish are more likely than non-infected fish to recruit young parasite.