The effects of food, predation risk and endogenous rhythmicity on the behaviour of juvenile plaice, Pleuronectes platessa L

The influences of prey (the bivalve Donax vittatus (de Costa)) and a predator (juvenile cod, Gadus morhua L.) on long-term behaviour patterns of juvenile plaice were investigated by time-lapse video-recording of behaviour in the laboratory. Analysis of the frequencies and durations of behavioural elements revealed two basic types of behaviour. In the absence of food, behaviour was dominated by elements associated with migration to and from the intertidal zone: Longer, less frequent swimming movements with occasional off-bottom excursions, especially around the predicted time of high water. Benthic activity was dominated by foraging behaviour when food was present and consisted of frequent short searching movements and occasional attacks on prey which continued throughout the tidal cycle, mixed with the migration component. Migration behaviour was strongly controlled by the endogenous tidally phased activity rhythm, whereas fish foraged only in the presence of food and independently of the phase of the tide. The presence of the predator depressed the expression of migration behaviour and reduced and delayed the onset of foraging, with greatly diminished feeding success. Expression of behaviour in the natural environment reflects the relative contributions of these exogenous and endogenous influences.