Diel movements of juvenile plaice Pleuronectes platessa in relation to predators, competitors, food availability and abiotic factors on a microtidal nursery ground

The distribution and movements of juvenile plaice Pleuronectes platessa and their potential predators and competitors were recorded in a small microtidal (similar to 20 cm) bay on the west coast of Sweden using underwater television and conventional netting techniques. There was generally close correspondence between the results obtained by the 2 methods. Young plaice moved upshore at dusk and returned to deeper water at dawn. Larger predatory fishes also moved upshore at night but not as far as the plaice; they also entered shallow water later and left for deeper water earlier than the plaice. One interpretation of these nocturnal upshore migrations by plaice, therefore, is that they minimise predation risk. Although stomach fullness was greater at night, the availability of suitable food items for the plaice throughout the bay makes it unlikely that the upshore movements were solely related to feeding. Because there was a strong diel temperature fluctuation in the bay, such movements also ensured that the fish remained in a fairly constant temperature. The main competitor of the plaice for food in July, the shrimp Crangon crangon, showed no such migratory movements and was mostly active during the day. The other dominant large crustacean (Carcinus maenas) was nocturnally active and did not markedly change its depth distribution over the 24 h period. The observed patterns of movement suggest that the timing of immigration and emigration is mainly triggered by changes in light intensity because only in C. maenas was any relationship found between activity and the phase of the tidal cycle.