During a mass prespawning migration, about 800 000 of S. scombrus enter the Gulf of St Lawrence through Cabot Strait over a few weeks in the spring. Although the strait is 105 km wide, mackerel migratory activity is concentrated in the first 6 km of nearshore waters on the southern side of the strait. Results suggest that mackerel use selective tidal stream transport to enter the gulf, although the vertical migrations through which selective tidal stream transport would be accomplished were not documented.