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The principles which govern horizontal motion induced by vertical migration in oscillatory, sheared (tidal) currents are discussed. A simple model which employs linear velocity shear in the vertical, and sinusoidal vertical migration, is used to establish the fundamental ideas. For linear shear. only when tidal and migration periods exactly match is long-term unidirectional transport possible. For non-linear forms of velocity shear, unidirectional transport is also possible for migration periods which are exact multiples of the tidal period. This may be regarded as a resonant interaction between tide and migration. For migration at non-tidal periods the horizontal displacement remains bounded but the amplitude of displacements increases as tidal and migration periods become close. The time between peak horizontal displacements is usually given by the beat period between tidal and migration periods.Die1 migration in M2 semi-diurnal tidal currents induces maximum horizontal excursions of no more than a few tens of kilometers and, over a duration of 15 d, there is no net displacement. In contrast diel migration in diurnal tidal currents of the K1 constituent (23.93 h) can lead to displacements of hundreds of kilometers and the duration of effective unidirectional transport is 171 d. A model due to Prandle (1982) which incorporates rotary tidal motion and more realistic velocity shear is presented as a tool for estimating migration-induced motion in tidal currents.

Norway lobster Nephrops norvegjcus (L.) larvae are hatched into the water column from areas of fine mud and spend about 50 d in the plankton before resettling onto mud as juveniles. Whilst in the pelagic environment, however, advection and horizontal turbulent diffusion spread larvae away from the mud region and thus potentially could limit recruitment. A simple advection-diffusionmortality model has been used to examine the likely losses of larvae from an N. norvegicus system due to pelagic dispersal. Turbulent diffusion alone causes insufficient loss to affect recruitment on mud patches larger than 20 X 20 km for reasonable estimates of eddy diffusivity. Low levels of mean advection (0.04 to 0.05 m S-'), however, severely limit settlement even on very large mud areas (100 X 100 km). Distributions of larvae released from a mud region In the western Irish Sea consistently show them to be spread southward away from the mud patch although it remains unclear whether advection in the region is sufficiently large for these losses to influence recruitment.

A mechanism is proposed by which net horizontal transport can be induced in an organism which migrates vertically in a purely oscillatory, vertically sheared tidal current. The mechanism contains two elements. First, for all reasonable tidal current profiles, net horizontal transport is induced in any organism which migrates vertically with a period which is an exact multiple of the tidal period. (This is the basis for the widelyknown process of selective tidal stream transport where the migration period and the tidal period are exactly equal.) The second element in the new mechanism is the observation that diel migration, the most common form of vertical migration, has a period of 24 h and is therefore an exact multiple of the principal solar semi-diurnal tidal constituent ($2) which has a period of 12 h. This relation between the $2 and diel migration periods stems from the fact that both phenomena are independently locked to the solar cycle. Diel vertical migration can therefore interact with the $2 tidal current constituent to induce longterm horizontal transport in migrating organisms. For reasonable tidal current and vertical migration parameters it is found that horizontal transport rates of 4 km d-1 are possible. The direction of net transport depends upon the phase of the $2 tidal current relative to local noon. The hypothesis is therefore proposed that geographical variation in $2 phase is a factor responsible for creating regions which are either retention-favorable or otherwise for diel-migrating marine organisms.