Effects of tidal streams on migrating Atlantic mackerel, Scomber scombrus L.

Abstract

During a mass prespawning migration, about 800 000 tons of Atlantic mackerel (Scomber scombrus L.) 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, as revealed by fisheries acoustics techniques and trawl catches, is concentrated in the first 6 km of nearshore waters on the southern side of the strait. Current-meters moored 2 km off the southern shore showed that tidal currents alternately enter and leave the Gulf with speeds up to 75 cm s⁻¹while current-meters moored at 6 and 11 km offshore measured surface currents leaving 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) were observed at slack tide. Mackerel densities increased at slack tides at the transition from a flow leaving to a flow entering the Gulf whereas they decreased at slack tides at the transition from a flow entering to a flow leaving the Gulf. Because times of slack waters were irregular, mackerel density increases did not occur on a regular semi-diurnal basis. These results suggest that mackerel use selective tidal stream transport to enter the Gulf of St Lawrence, although we were not able to document the vertical migrations through which selective tidal stream transport would be accomplished. This is the first report suggesting selective tidal stream transport in fast-swimming pelagic teleosts.