Effect of light intensity on activity and food-searching of larval herring,Clupea harengus: a laboratory study

Larvae of Clupea harengus were reared from spawning herring caught in March 1982 and 1983 in the Firth of Clyde, Scotland. An infra0red observation technique was used to record the behaviour of larval herring both in shallow dishes using a top view and in a tank 2 m deep using a side view. The amount of time larvae spent swimming, which was minimum in complete darkness, increased with increasing light intensity and as the larvae grew. Maximum swimming speeds of feeding larvae were recorded at light intensities between 10 and 100 lux. The presence of food organisms (Artemia sp., Brazilian strain) at light intensities below the feeding threshold (0.1 lux) caused an increase in the proportion of time spent active, but light intensities above the threshold had different effects, depending on developmental stage: larvae of 12 mm increased swimming speed, but 21 mm larvae decreased speed. In the 2 m deep tank in darkness, larvae displayed inactive periods wherein they sank head first, interspersed with periods of upward swimming. As light intensity increased, vertical swimming was replaced by horizontal swimming. These results are discussed with reference to food searching and vertical migration of larval herring in the sea.