The population of sand smelt, Atherina presbyter, breeding in the Fleet, Dorset, shows a high infection of diplostomiasis. The population was studied in 1983 to clarify aspects for this parasitic condition previously analysed for the sand smelt population at Fawley, Southampton Water. All age classes showed a higher percentage infection and mean number of metacercaria per fish than at Fawley. Analysis of postlarvae and juveniles showed that infection can occur at 1 week old, and verified the hypothesis that the scales of older fish inhibit cercarial settlement. Circumstantial evidence suggests that Hydrobia ventrosa may be the first vector host for this parasite, and the densities of this species and of nesting little tern colonies would account for the differential infection between these two sand smelt populations. The increase in infection of $2 fish at Fawley cannot be attributed to mixing with the Fleet population, and the different infection levels demonstrate population isolation. High levels of infection are limited to this part of the English channel; sand smelt samples from around the coasts of the British Isles show minimal infection rates elsewhere and suggest a southerly distribution of the parasite, away from Atlantic oceanic waters.

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