Danish fishing vessels can be characterized by their diversity of fishing practice in terms of fishing gear and target species, and by their operational flexibility in respect of these fishing practices throughout the year. We describe the temporal fluctuations in this flexibility by following the activity of individual fishing vessels between 1989 and 2001. Initially, a typology of fisheries (classification of fishing trips) and vessel groups (classification of fishing vessels) was established through multivariate analyses of catch and effort data for 1999. In all, 54 fisheries and 25 vessel groups were identified. These typologies were then applied to all data for the whole time period, and the dynamics of fisheries and vessel groups investigated. The dynamics of vessels groups are studied both within groups (main and secondary fisheries, changes in activity patterns) and between groups (tracking of vessels shifting between groups). Results show average stability of vessel activity in terms of the main fishery, along with a great diversity of secondary fisheries and some possibilities for shifting between gears and areas. We conclude that the level of technical interactions is high, and that separation into distinct management units is difficult.