TURBULENCE AND ICHTHYOPLANKTON - INFLUENCE ON VERTICAL DISTRIBUTIONS AND ENCOUNTER RATES

Two different aspects of the effects of turbulent mixing on eggs and f ish larvae are considered here. In the first topic the effects of phys ical processes on the vertical distribution of the eggs are considered . The physical processes which determine the vertical distribution of fish eggs, and all other immobile plankton, are the buoyancy of the pl ankton and the vertical mixing of the water column. Analytical models are presented to demonstrate the influence of the various terms. A num erical model is also applied to show the effects of non-stationary sol utions. The model results are compared with field measurements. In the second topic the effects of turbulence on plankton encounter rates ar e considered. The processes are illustrated by field data on first-fee ding cod larvae feeding on Calanus nauplii. These field data show that wind-induced turbulence strongly enhances the feeding rate of the lar vae. For first-feeding cod larvae the feeding rate increases by a fact or of about 7 when the wind speed increases from 2 m s(-1) to 10 m s(- 1). Model simulations show how the influence of turbulence-enhanced en counter rate decreases as the larvae grow, increase their swimming spe ed and switch to larger prey of lower concentration. The simulations s how that the turbulence-induced encounter rare decreases exponentially as cod grow, but significantly influences feeding until the stage of 2 months old juveniles.