Population biology of eyeflukes in fish from a large fluvial ecosystem: the importance of gulls and habitat characteristics

Spottail shiners (Notropis hudsonius) were monitored for eyeflukes monthly at four sites in the St. Lawrence River, Quebec, from spring through autumn in 1997 and 1998. In general, mean abundance of Diplostomum spp. in the le ns of spottail shiners was highest at sites near large ring-billed gull (La rus delawarensis) colonies and was higher in 1998 than in 1997. Population studies demonstrated a major period of recruitment in the late summer - ear ly autumn. Mean abundance increased between November and May, when sampling was logistically not possible, reflecting late-fall or early-spring recrui tment. Golden shiners (Notemigonus crysoleucas) and small yellow perch (Per ca flavescens) were monitored at a single site (Ile aux Ours) in 1997 and 1 998. Mean abundance of Diplostomum spp. in the lens of golden shiners at Il e aux Ours was higher in 1998 than in 1997. Recruitment occurred in the ear ly summer and between November and May, suggesting late-fall or early-sprin g transmission. Mean abundance decreased between August and October in both years. Yellow perch at Ile aux Ours were infected with four different para site species in their eyes. Mean abundance of Diplostomum spp. in the vitre ous humour or retina increased during the summer and fall and then decrease d between the fall and following spring. Diplostomum spp. in the lens incre ased in abundance in the early summer, and then decreased during the fall. Both Tylodelphys scheuringi and Neascus spp. in the vitreous humour were mo re abundant in 0+ than in 1+ perch. Mean abundance of T. scheuringi increas ed over the summer in 1997 and then dropped to zero in the same cohort over winter, suggesting that the parasite lives for only 1 year. Neascus spp. o nly were found in 1998. Distributional data, together with seasonal informa tion, suggest that habitat characteristics contribute significantly to the abundance of Diplostomum spp. along the St. Lawrence River. Proximity to gu ll colonies enhances abundance, but higher rates of predation at wetland si tes depress abundance and restrict the accumulation of parasites.