Improving time budget estimates through the behavioural interpretation of dive bouts in harbour seals

• How does dive bout structure translate into activity budgets in harbour seals?
• Dive bouts and surface periods mainly reflect the altering of activity and resting.
• Resting occurs also as low-activity diving, driven by negative buoyancy.
• The cumulative patterns of different resting forms follow the daylight cycle.
• Activity periods may reflect temporal resource variation better than entire trips.

The study of the organization of dives in bouts has provided insights into the foraging ecology of many species of diving animals. In otariids and seabirds, the alternation between dive bouts and periods at the surface has been proved to reflect the distinction between time spent foraging in patches and moving between them, allowing bouts to be interpreted as foraging units. Phocid seals, however, are known to be able to travel and rest while diving, making the interpretation of bouts less straightforward. We investigated the behavioural interpretation of dive bouts and surfacing periods in harbour seals, Phoca vitulina. We propose an approach in which the organization of bouts of dives at different temporal scales is compared with information on the seals' horizontal movements and dive characteristics. We found that prolonged surfacing periods could be generally interpreted as at-sea resting events. We also examined the occurrence of potential resting behaviour while diving and found that harbour seals perform low-activity dives, with different incidence between individuals. We conclude that bout organization in harbour seals mainly reflects the alternation of periods of activity and resting and that harbour seals spend a significant amount of time resting at sea. However, dive bouts cannot be considered as behavioural units, since both movement and some of the resting activity occur within them. This approach highlights the presence in this species of behaviours of similar function (i.e. resting), expressed in different forms (i.e. haul out, prolonged surfacing, low-activity diving), the cumulative patterns of which are well related to the daylight cycle.