Effect of a trematode infection on growth, reproduction, and mortality of shorthead sculpin

Shorthead sculpin ( Cottus confusus Bailey and Bond, 1963) in Birch Creek, Idaho, were infected with the trematode Euryhelmis cotti (Simon, 1972). Our objective was to determine if intensity of infection had an influence on growth, reproduction, and mortality of shorthead sculpin at two sites in Birch Creek. Prevalence of trematode infection was 100 % at the upstream site compared to 32 % prevalence at the downstream site; intensity of infection was at least 15 times greater in shorthead sculpin at the upstream site. von Bertalanffy growth models differed significantly between sites; the growth coefficient (K) was higher for sculpin at the upstream site. Gonadosomatic index for both males and females was significantly higher for sculpin found upstream. Females at the upstream site also had significantly more eggs per clutch compared to females at the downstream site. Annual mortality was significantly higher upstream (A = 57 %; 95 % CI = 52–62 %) compared to downstream (A = 42 %; 95 % CI = 38–46 %). Consequently, the density of sculpin was lower at the upstream site (13 fish/100 m 2 ) compared to the downstream site (54 fish/100 m 2 ). We did not find evidence to suggest that growth and reproduction of shorthead sculpin were influenced by infection of E. cotti , but rather were influenced primarily by density dependent factors. Mortality was higher for shorthead sculpin at the upstream site with high prevalence and intensity of infection. However, we do not know if higher mortality was a direct consequence of trematode infection or merely correlated with trematode infection.