Tidal Exchange of Anadromous Rainbow Smelt (Osmerus mordax) Larvae Between a Shallow Spawning Tributary and the St, Lawrence Estuary

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Abstract
The factors involved in the riverine transport and tidal exchange of yolk-sac rainbow smelt (Osmerus mordax) larvae between Ouelle River and the St. Lawrence estuary were documented. A 72-h sampling series provided evidence of synchronous nighttime hatching of larvae independent of water temperature and river hydrodynamics. Although light conditions influence the vertical distribution of yolk-sac larvae at the river's mouth, the influence of current speed on larval vertical distribution appears to be a more important element in the dynamics of riverine drift and tidal exchange. Yolk-sac larvae sink to the bottom as water currents decrease prior to low slack water. With the intrusion of estuarine water along the bottom during the flood, yolk-sac larvae drifting from the river appear to be mixed with larger post yolk-sac larvae of estuarine origin resulting in temporary retention at the mouth of Ouelle River. However, longer ebb phases in the river result in a net transport of smelt larvae towards the St. Lawrence estuary. We conclude that the riverine transport and mixing of yolk-sac larvae with the estuarine water mass is controlled principally by the hydrodynamics of the system and the nighttime hatching and specific gravity of yolk-sac larvae.