Abstract
Winter flounder, Pseudopleuronectes americanus, caught in Passamaquoddy Bay, New Brunswick, Canada, and held captive under crowded conditions, developed mixed infections of Trichodina murmanica Polyanski, 1955 (Ciliophora) and Gyrodactylus pleuronecti Cone, 1981 (Monogenea). A protocol involving sequential sieving was used to separate the two species of parasites and produce viable experimental baths. Replicate groups of juvenile, hatchery‐reared flounder received one of the following treatments: mixed bath of G. pleuronecti and T. murmanica , bath of G. pleuronecti, bath of T. murmanica or parasite‐free (controls). The abundance of both parasites correlated negatively with condition factor of the flounder ( r =−0.354, P Trichodina ; r =–0.205, P Gyrodactylus ). During the periods of peak parasite abundance (1–2 weeks postinfection), the effect of the two parasite species was additive, as mean condition factor and the percentage change in weight were significantly lower (ANOVA, P