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 Arternia 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 tlme 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 swlmming was also seen at this time in the light. Arlemia 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.
Multiple random quadrats (25cm by 25cm) were sampled at five sites on depositing shores in South Wales and the Isle of Man. The effect of increased number of samples was examined in relation to the reliability of estimation of various ecological measures - J, H', number of species, total individuals, total individuals of selected species. The replication required varies with the measure involved, but generally 10–20 or even more quadrats are needed for reliable information which can be used to detect other than very gross changes with time. In monitoring programmes it is thus advantageous to concentrate sampling effort on a limited number of sites with high replication because of this considerable spatial variation. The need to define specific aims, and to design a sampling format related to these aims, is emphasized.
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