Forty-five percent of 306 brown trout from 16 Montana streams were infected with one or more of the nematodes Cystidicoloides salvelini, Bulbodacnitis globosa, Rhabdochona sp., and Eustrongylides sp. The relationships between incidence and intensity of nematode infections and age and sexual maturity of the host fish were studied. Generally, sexually mature female brown trout had a higher rate of infection and had more nematodes per infected fish than immature female brown trout. Higher incidence and intensity of infection in sexually mature fish was attributed to more aggressive feeding behavior leading to more exposure to the intermediate hosts (mayflies) of the nematode parasites.
Lockard, Lawrence L.; Parsons, R. Randall; and Schaplow, Barry M.
"Some relationships between internal parasites and brown trout from Montana streams,"
Great Basin Naturalist: Vol. 35
, Article 13.
Available at: http://scholarsarchive.byu.edu/gbn/vol35/iss4/13