EFFECTS OF TIDAL STREAMS ON MIGRATING ATLANTIC MACKEREL, SCOMBER-SCOMBRUS L

During a mass prespawning migration, about 800 000 tons of Atlantic ma ckerel (Scomber scombrus L.) enter the Gulf of St Lawrence through Cab ot Strait over a few weeks in the spring. Although the strait is 105 k m wide, mackerel migratory activity, as revealed by fisheries acoustic s techniques and trawl catches, is concentrated in the first 6 km of n earshore waters on the southern side of the strait. Current-meters moo red 2 km off the southern shore showed that tidal currents alternately enter and leave the Gulf with speeds up to 75 cm s(-1) while current- meters moored at 6 and 11 km offshore measured surface currents leavin g the Gulf throughout the tidal cycle. The largest mean (1.2 kg m(-2)) and maximum (16.3 kg m(-2)) mackerel densities (from 11-80 m deep) we re observed at slack tide. Mackerel densities increased at slack tides at the transition from a how leaving to a flow entering the Gulf wher eas they decreased at slack tides at the transition from a flow enteri ng to a flow leaving the Gulf. Because times of slack waters were irre gular, mackerel density increases did not occur on a regular semi-diur nal basis. These results suggest that mackerel use selective tidal str eam transport to enter the Gulf of SI Lawrence, although we were not a ble to document the vertical migrations through which selective tidal stream transport would be accomplished. This is the first report sugge sting selective tidal stream transport in fast-swimming pelagic teleos ts. (C) 1995 International Council for the Exploration of the Sea