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Published online by Cambridge University Press: 11 May 2009
Vertical migration interacts with oscillatory tidal flows to produce horizontal transport. When the vertical migration period is an exact multiple of the tidal period then net uni-directional horizontal transport can occur. This happens most effectively in ‘selective tidal-stream transport’ for which vertical migration into the flow is specifically synchronized with the dominant (usually M2) tidal constituent. When migration and tide are not synchronized there is no net transport and, instead, a horizontal displacement oscillation of the organism takes place at the beat period between the vertical migration and tidal periods. The most common form of vertical migration is synchronized, not with the tide, but with the day-night cycle. Diel migration in M2-period tidal currents induces no net transport, but can produce excursions of several tens of kilometres over just a few days. Diel vertical migration can, however, interact with the sun-generated part of the semi-diurnal tide to produce net horizontal transport, the direction of which is controlled by the phase of the S2 tidal currents. The spatial distribution of phase implies that regions of horizontal convergence and divergence will result from diel migration. Diel migration in diurnal-period tides causes the preferred direction of transport to reverse at six-monthly intervals.