Diplostomiasis in the sand smelt, Atherina presbyter (Cuvier), population at Fawley Power Station

Diplostomiasis, or black spot, is a disease of teleost fish, which results from the encystment of metacercarial larvae of digenean trematodes in the skin of the fish. This parasite occurs on sand smelt, Atherina presbyter , and pout, Trisopterus luscus , at Fawley. Since the debilitating effects of disease or parasitism may be a factor influencing fish impingement, the incidence of black spot in the Fawley sand smelt population has been studied. Fish 3 months old can be infected, and an average of 9% of the population up to 2 years old is infected. There is a rapid size-correlated rise in infection in 2 + fish, associated with their migratory habit. The metacercarial morphology has been examined, and it is tentatively identified as a species of Neodiplostomum . There is a greater settlement of metacercaria on the fins and the eye of the sand smelt than on the scaled parts of the body. No seasonality or change in levels of infection over the years 1977 to 1981 were found. The incidence of the parasite has no effect on the impingement of sand smelt at Fawley; the reasons for this are discussed. Models of the changing rate of infection with migration imply that the population associated with the power station is less infected than surrounding natural populations. The infection rates of diplostomiasis in separate populations of host fish may prove useful in distinguishing the origins of individuals in a mixed impingement on the power station screens.