Foraging behaviour of larval cod (Gadus morhua) influenced by prey density and hunger

Fish larvae meet diverse environmental conditions at sea, and larval growth and chance of survival depend on a flexible response to environmental variability. The present study focuses on the flexibility of the foraging behaviour of larval cod in a series of laboratory experiments on larval search activity, prey selectivity, and hunger in a variable prey environment. Gadus morhua eggs were collected in March 1992 and 1993 from the Kattegat area, Denmark, fertilised and incubated in the laboratory. After hatching, the larvae were transferred to rearing tanks of 172 litres. The behaviour of larvae (6 to 7 mm long) was observed visually, and prey attacks, swimming activity and gut contents were registered across a range of 1 to 120 copepod nauplii l^-1. When prey density decreased, larvae increased their swimming activity, increased their responsiveness to prey (distance of reaction) and decreased their prey size selectivity. Behavioural response was to a large degree determined by the level of hunger, represented by the number of newly ingested prey in the gut. The findings show that cod larvae have a flexible response to changes in feeding conditions and imply that larvae can grow and survive even in the lower range of (mean) prey densities measured at sea.