| dc.description | The displacement of the pelagic early life stages of fish is an important component of the theories, hypotheses and concepts concerning the natural regulation of fish populations. However, empirical estimates of the magnitude and frequency of the displacements are difficult to obtain. Therefore estimates are often made using indirect methods. In this thesis some of the components needed for the development of displacement indices of haddock (Melanogrammus aeglefinus L.) eggs are investigated. Haddock and cod year-class sizes, estimated from the virtual population analyses of 3 haddock and 6 cod populations, are shown to be only weakly correlated within species and uncorrelated between species, thus indicating that population specific processes are the largest contributor to recruitment variation. An examination of literature estimates of egg abundances, egg development times measured at constant incubation temperatures and time series of sea surface temperatures shows that haddock spawning times and the age of eggs at hatching vary inter-annually and between populations. Discrete-depth ichthyoplankton samples taken over Browns Bank show that the vertical distribution of haddock eggs varies ontogenetically. A one dimensional advection-diffusion model, developed to explain the observed vertical distributions, shows that the horizontal transport of haddock eggs in a surface Ekman layer is overestimated by indices of surface velocities. A kinematic model of a constant, depth-independent Eulerian gyre illustrates the non-linear relationship between the displacement of eggs spawned inside the gyre and wind speed, when the wind is assumed to induce a horizontally uniform flow superimposed on the gyre. Finally, a two-dimensional particle tracking model for estimating the displacement of haddock eggs spawned over Browns Bank is developed. A comparison between the trajectories of drogues released over Browns Bank with those estimated with the model indicates the model is unable to reliably estimate drogue trajectories. | en_US |
