The influence of tidal streams on the pre-spawning movements of Atlantic herring, Clupea harengus L., in the St Lawrence estuary

Eight Atlantic herring (Clupea harengus L.) implanted with ultrasonic transmitters were tracked to determine their movements during the pre-spawning period in the tidally energetic, upper St Lawrence estuary. Herring released near the south shore travelled upriver near the shore while herring released near an island where significant spawning occurs remained in the vicinity of that island. Herring near the shore moved upriver regardless of the tidal phase while herring close to the island swam actively against tidal currents during both phases of the tide. Herring near the coast may have been migrating to an alternate spawning site further upriver while herring near the island remained in close proximity to the island's spawning site as a result of their behaviour. Herring travelling near the coast seemed to take advantage of tidal currents by swimming with tidal streams when the tide flowed in the migratory direction (flood tide) and by swimming against them when the tide flowed counter to it (ebb tide). These fish gained more ground during the flood tide than they lost during the ebb. A circular statistical analysis of movements-through-water showed that fish were oriented against the flow during the ebb phase. Selective tidal stream transport was not observed since there was no evidence of vertical migrations associated with tides.