Abstract
Diurnal vertical movements of Cancer pagurus L. have been studied by skin-diving surveys on an exposed rocky islet on the Norwegian Skagerrak coast. The site studied was bounded in depth by the upper limit of Laminaria digitata (Hudson) Lamouroux (3-5 m) and the surface. The investigation was carried out between March and December 1980. Observations clearly showed that C. pagurus prefers to forage in shallow water at night, and crabs were observed on all nights ofthe survey from late May to December. Temperatures below 5°C in the upper water layers seem to prevent crabs moving to the shore. There were statistically significant differences in the number, sex and size distribution of the crabs throughout the investigation period. This was probably due to moulting and mating. Soft -shelled crabs were observed from early August until the end of the year. Tagging of crabs showed that males were stationary, whereas females migrated over longer distances.