Seasonal and diurnal activity of sixgill sharks (Hexanchus griseus) on a shallow water reef in the Strait of Georgia, British Columbia

The bluntnose sixgill shark (Hexanchus griseus) is a large, deepwater, cosmopolitan species whose range includes the Strait of Georgia, British Columbia. Although this species is probably the top predator on most of the world's continental shelves, there is little information on its population biology or behaviour. At a unique site off Flora Islets, in the Strait of Georgia, a number of H. griseus individuals show unexpected diurnal activity on a shallow reef. Regular movements of these sharks along the base of the reef wall allowed us to use an automated, time-lapse video system to make the first extensive observations of their behaviour between June 2001 and July 2002. Sixgill shark frequency (no. of sharks observed/h) increased in June and reached a peak between mid-June and mid-July. Sixgill sharks were not observed between October and May. Sixgill shark frequency was highest in the afternoon and the sharks tended to move as individuals rather than in groups. The observations suggest that this shallow-water activity is not related to either reproduction or feeding and its purpose remains unclear.