Abstract
Vertical and horizontal movements of intact and bilaterally enucleated herring larvae, Clupea harengus, were monitored during changes in light intensity. The positive and negative phototaxes exhibited by intact larvae in a horizontal light beam are absent with enucleated larvae. Vertical movements to an overhead light source were exhibited with both groups swimming up the column at surface light intensities below 10‐3 lx (vertical migration) and intact larvae also rising in the water column at surface light intensities above 10‐1 lx (positive phototaxis/positive photo‐orthokinesis). The vertical migration response is relatively unaffected by water depth. Vertical migration may be driven continuously by repeated light cycles. The larvae exhibit no clear circadian rhythms under constant light or darkness nor diel rhythms of sensitivity or responsiveness to changes in light intensity.
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