Partitioning the influence of physical processes on the estimation of ichthyoplankton mortality rates. 1. Theory

This paper examines the effects of natural spatial-temporal variability in ichthyoplankton concentration and in currents on the estimation of mortality rates. We derive expressions for the biases and variances of mortality estimates computed from the change in plankton concentration between successive surveys as well as estimates of the corrections due to the advection of plankton. We demonstrate that estimate bias depends primarily on how well the mean current and mean ichthyoplankton fields are sampled whereas the variance depends on the variability about the mean in currents and plankton concentration and on the time and length scales on which this variability occurs. Simplified versions of the theoretical expressions provide an easily implemented framework for evaluating the quality of field sampling plans.