Abstract

Abundances of eye flukes (Diplostomum spp.) were compared between walleye (Stizostedion vitreum) and white suckers (Catostomus commersoni) collected in late summer 1997 from Lake St. Louis and Lake St. Pierre, two expansions of the St. Lawrence River in Quebec, Canada. The white sucker, a benthic consumer, was more heavily infected than the walleye, a pelagic piscivore, in both lakes. Infection levels increased significantly with host age and size. For both species, abundance of Diplostomum spp. within each age group and length class was higher in fish from Lake St. Louis than in those from Lake St. Pierre. Walleye of all ages and white suckers 7 years old from Lake St. Louis were also larger at age than those of corresponding age from Lake St. Pierre. Therefore, walleye and white suckers from Lake St. Louis are probably different populations from those in Lake St. Pierre. The higher infection levels in Lake St. Louis are most likely due to the larger number of ring-billed gulls (Larus delawarensis), an important definitive host of Diplostomum spp., in colonies in close proximity to that lake; there are >75 000 pairs within 40 km of Lake St. Louis and 16 000 pairs within 40 km of Lake St. Pierre. No detrimental effects of infection with Diplostomum spp. could be detected on fish fork length, body mass, condition index, or gonadosomatic index. Walleye from shallow lentic waters in Lake St. Louis were larger and possessed heavier infections of Diplostomum spp. than those from deeper lotic waters. Walleye collected from a fixed trap near Quebec City in July 1997 were smaller but more heavily infected with Diplostomum spp. than those collected in October, which implies that different populations of fish may be present seasonally at this location. A visual index developed to measure the degree of opacity of the lens of fishes does not appear to be a reliable indicator of levels of infection with eye flukes. Experimental infection of laboratory-raised juvenile ring-billed gulls with metacercariae from the lenses of various fish species collected in the St. Lawrence River demonstrated that metacercariae were primarily Diplostomum indistinctum (84–92%), the remainder being Diplostomum huronense, and this pattern is consistent across host species and localities.