| In fish populations, starvation, dispersion and predation are responsible for early life-history mortality. Endoparasitism can potentially affect those factors. In the St-Lawrence Middle Estuary, a population of copepods transfers an intestinal parasite to rainbow smelt larvae (Osmerus mordax). These larvae accomplish vertical migrations to achieve retention in the estuarine turbidity maximum (ETM) and parasitism is suspected to affect this behavior as well as their growth-rate. To evaluate these hypotheses, we collected larvae in summer of 1996 and 2001, before and after the arrival of the parasite in the ETM. Individual growth trajectories were reconstructed using otolith microstructure. The index of parasitism is not significatively different between larvae found in the bottom and upper layer during the ebbing or flooding tide indicating that parasitism does not directly affect the vertical distribution of the larvae. However, parasitism affects growth rate and thus may reduce the survival of larvae. Parasitism had a significant impact on size-selective mortality with smaller infected larvae showing the greatest mortality. Changes in the dynamics of the infection may thus impact smelt recruitment. |
