FORAGING TIME STRATEGY OF SMALL JUVENILE PLAICE - A LABORATORY STUDY OF DIEL AND TIDAL BEHAVIOR PATTERNS WITH ARTEMIA PREY AND SHRIMP PREDATORS

The natural pattern over time of foraging and migration in juvenile plaice Pleuronectes platessa has evolved to enhance the survival of the fish. It is a product of endogenous rhythms and direct responses to biological and physical conditions, including their predators and prey. This study aimed to determine the relative roles of these influences through experimental manipulation of the physical and biological conditions. Infrared television cameras and illumination were used to observe freshly caught juvenile plaice in the light and dark over 24 h periods in arena tanks. Single Crangon crangon and Artemia nauplii were added to these tanks as predators and food respectively. The behaviour of juvenile plaice in the light was dominated by swimming on the sand surface, with little activity on the bottom during darkness. Most swimming on the bottom occurred 0 to 2 h after the expected time of high water. Activity in darkness largely comprised swimming in the water column or at the surface, with most swimming seen 2 to 4 h after high water. Off-bottom swimming was also seen at this time in the Light. Artemia as prey reduced swimming on the bottom during the light period, but had no effect on off-bottom activity. The presence of single C. crangon in the tank had no discernible effect on behaviour. Different functions are proposed for pelagic and benthic swimming. Both may be used for migration while pelagic swimming may avoid predators on the bottom at night. Benthic swimming may be used for foraging.