EFFECTS OF ENDOGENOUS RHYTHMS AND LIGHT CONDITIONS ON FORAGING AND PREDATOR-AVOIDANCE IN JUVENILE PLAICE

The influences of a light : dark cycle and a persistent endogenous rhythm of activity on foraging (on the bivalve Donax vittatus) and avoiding a predator (juvenile cod, Gadus morhua) were investigated in freshly-caught juvenile plaice, Pleuronectes platessa. Time lapse video recordings were made of fish in the presence and absence of prey and predators in laboratory tanks over 24-hour periods between the times of successive daytime low waters. Endogenous rhythms of activity were seen in all experimental treatments. Swimming both close to the bottom and in the water column showed a strong circatidal rhythm, with most activity 2 to 3 h after the predicted time of high water. Swimming in the water column was more frequent at night than by day. In the presence of a population of Donax, whose siphon tips could be eaten as food, swimming close to the bottom became more frequent. This increase in benthic swimming was independent of the endogenous cycle of activity and was correlated with the frequency of : attacks on siphons. The presence of the cod predator delayed the onset of foraging activity, producing a foraging/predator avoidance trade-off. The independence of foraging from light and endogenous rhythms suggests that this trade-off may be similarly independent. The cod also greatly reduced swimming in the water column in darkness, behaviour apparently unrelated to foraging.