Monitoring the Vertical Movements and Light Responses of Herring and Plaice Larvae

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Abstract
The movements of herring and plaice larvae of different age, from hatching to about 8 weeks old, were monitored at different levels in a vertical tube by means of matched pairs of thermistors controlled and ‘balanced’ by bridge circuits. Movement of larvae past the thermistors upset the balance and the resulting change in the circuit could be followed on a pen-recorder. Having established, by visual comparisons, the interpretation of the pen record, experiments could be performed over extended periods or at low light intensities when visual observations would have been tedious or impossible. The numbers of larvae present were shown by the number of spikes on the pen record per unit time, while a measure of their size or activity was given by the height of the spikes.The light intensity could be altered by interposing neutral-density filters between an artificial light source and the surface of the tube. Alternatively, natural daylight could be beamed in by a mirror above the tube when placed by a window. Using artificial light, changes of very large intensity had little effect on the larvae until a critical level was reached (equivalent to the intensity at late dusk or early dawn) when a vertical migration took place. A similar vertical migration took place when the natural light at dusk was beamed in; many larvae then moved to the surface and remained there, apparently very active throughout the night untill the light increased at dawn. Vertical migration could be simulated in the laboratory at any time by imposing an artificial light-cycle of appropriate amplitude on the larvae.