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In the St. Lawrence middle estuary, the larvae and juveniles of rainbow smelt Osmerus mordax are infected with intestinal parasites (the cestode Proteocephalus tetrastomus). Such para- sitism is suspected to impair growth rate, which may in turn influence the ability of smelt to migrate vertically and to be retained in the estuarine turbidity maximum (ETM), the nursery area of this population. We hypothesised that parasitism contributes to size-selective mortality by altering patterns of vertical migration, thus contributing to transport losses from the nursery area. Parasitism was found to affect growth rate, as determined by otolith microstructure analysis, and to contribute to mortality. Larvae which survived the parasitic infection were found to be, on average, larger than the uninfected survivors at some point in their early life history prior to infection. Significant size- selective mortality was observed among infected larvae relative to uninfected larvae. There was, however, no significant difference in the vertical distribution of parasitised versus non-parasitised larvae, indicating that size-selective mortality was not due to transport losses from the nursery area. Rather, starvation or predation was most likely responsible for observed mortality patterns. We illustrate that variability in the incidence of parasitic infection among years may form the basis for variability in recruitment and the abundance of the adult smelt population.

Cestode parasitism as a regulator of early life-history survival in an estuarine population of rainbow smelt Osmerus mordax