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0-group plaice were caught in Red Wharf Bay, Anglesey, North Wales, tasged with coloured liquid latex, and released in the bay or, after transplantation, at a position on a sandbank 3.5 krn offshore. The pattern of movement shown by recaptures of plaice caught and released at the same position in the bay was one of extremely restricted movement of less than 0 5 km over several months, with only a slight interchange of fish between the sides of the bay and the centre. These restricted movements contrasted sharply with relatively long distances travelled by fish displaced to an unusual depth and position offshore and to a position at the other side of the bay 3 km away. In the first case a high proportion returned rapidly to the usual depth preference inshore and in the second a recapture pattcrn significantly different from that of control fish was demonstrated. Both results indicated a return to a preferred or familiar situation.

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Whiting collected from 36 stations in the North Sea in August 1985 and February 1986 were examined with reference to the prevalence, intensity and microhabitat utilization of three ectoparasites, namely hrnueocera brunchialis (Copepoda: Pennellidae), Clavella adunca (Copepoda: Lernaeopodidae) and Diclidophoru merlangi (Monogenea: Polyopisthocotylea). Maximum prevalences and mean intensities, at any one station, for the three parasites were 74,45.8,60% and 1.07,0.56 and 1.30, respectively.At the stations sampled, infestation levels with L. branchiulis and C. udunca were significantly higher in the winter than in the summer, with winter mean parasitic intensities being between five and eight times higher than those in summer. A converse situation applied with D. merlangi, with summer mean intensities being over four times larger than those in winter.Pooled winter and summer parasitic infestation data, from the 36 stations, enabled large-scale zoogeographical parasitization patterns to be recognized. Both L. branchialis and C. udunca are rarely seen on whiting in the North Sea north of latitude 56" N. South of this latitude, prevalences and intensities of L. branchialis are higher on the eastern than on the western side of the sampling zone, while those of C. adunca are similar on both sides of the North Sea. D. merlungi's infestation exhibits a striking latitudinal cline in the study area, with prevalences and intensities increasing steadily from south to north.All three ectoparasites were non-randomly distributed within the whiting gill and/or buccal cavities and showed marked microhabitat preferences. Possible explanations for the seasonal and geographical variations in the parasitic infestations are discussed.