WINTER GEOMORPHOLOGICAL PROCESSES IN THE SAINTE-ANNE RIVER (QUEBEC) AND THEIR IMPACT ON THE MIGRATORY BEHAVIOR OF ATLANTIC TOMCOD (MICROGADUS TOMCOD)

Knowledge of the migratory pattern of the Atlantic tomcod (Microgadus tomcod) towards it spawning site is scanty and controversial. This sit uation is partly a result of the fact that the migration takes place d uring winter, a period when it is difficult to investigate the relatio nships between flow dynamics, habitat and fish behaviour. The data rep orted in this paper suggest that channel morphology and flow dynamics, controlled both by the tidal regime and the ice cover, influence the migratory behaviour of the Atlantic tomcod population of the Sainte-An ne River (Sainte-Anne-de-La-Perade, Quebec). Measurements of channel m orphology and how velocity at low water indicate that the formation of ice cover in winter, combined with the presence of sand bars near the confluence of the Sainte-Anne with the Saint-Laurent River, causes a decrease in the cross-sectional area of the channel. This reduction in area is compensated for by an increase in flow velocity at the mouth of the river, when velocities larger than 30 cm/s were measured at low water. Underwater video observations of tomcod movements in the Saint e-Anne River indicate that such how velocities limit access of upstrea m migrating fish to the spawning site. The data demonstrate that upstr eam fish migrants avoid the downstream flow velocities occurring durin g the falling tide and favour the short period of flow reversal associ ated with large rising tides in order to move upstream. (C) 1998 John Wiley & Sons, Ltd.