Size susceptibility to trematode-induced mortality in the endangered fountain darter (Etheostoma fonticola)

We determined infestation rates for dead and surviving larval, juvenile, and adult fountain darters (Etheostoma fonticola) exposed to an exotic trematode, Centrocestus formosanus. Estimates from survival curves indicated that 50% of the larval fish (total length 9 to 13 mm) would die within 116 minutes and 50% of the juvenile fish (16 to 20 mm) within 330 minutes when exposed to approximately 555,000 cercariae/L. Less than 25% of the adult fish (36 to 41 mm) had died in 8 h of exposure. Number of metacercariae that caused mortality was directly related (P < 0.01) to fish length, although length alone did not explain the accelerated rate of mortality observed in smaller fish. Varying degrees of trematode tolerance among size groups suggested that larval and juvenile fish were more susceptible to mortality induced by C. formosanus infection than adults. This result has implications for population health as greater mortality in smaller fountain darters could limit the number of fish reaching sexual maturity.