Herring (Clupea harengus) filter-feeding in the dark

The behaviour of juvenile herring (Clupea harengus L.) feeding on Artemia sp. nauplii in both light and dark was recorded using an infra-red-sensitive television-recording technique. In the light, two modes of feeding were observed, particle biting and filtering, but in the dark only filtering was observed. Marked differences in swimming behaviour were seen between light and dark. In the light, the fish continued to school while feeding in both modes; in the dark, the school dispersed, the fish swam slower in tight circular paths and fed only by filtering. In the dark, filtering fish swam faster (0.11 m s^-1) than non-filtering fish (0.07 m s^-1). In the light, no difference in speed was measured between filtering and non-filtering fish (0.34 m s^-1). Owing to the lower filtering speeds in the dark, the removal rate of nauplii from the water was much lower than in light, except in the highest prey concentrations. This suggests that if night-time filter-feeding takes place in the sea, it will be of importance only when exploiting dense patches of food.