| Summary: | The haul-out behaviour of harbour seal (Phoca vitulina vitulina) is influenced by several factors such as the tidal state and environmental variables. Understanding these effects is important for designing counting-surveys providing data necessary to be able to estimate population size. The haul-out behaviour of harbour seals was investigated during the moulting period in three different localities by performing repeated land based visual counts at haul-out sites. The results from the counts were modelled using generalized additive mixed modelling to gain a better understanding of the relationship between the fine scale haul-out behaviour of harbour seals and the tidal cycle, as well as other sources of variability affecting the number of seals hauled out. In addition, results from aerial survey photographs of harbour seals from the same areas were compared to the results from the land based counts. The development of hauled out seals in time at haul-out sites was explained by the tidal cycle and other sources of variation on haul-out behaviour such as disturbance, time of day and movement of seals between haul-out sites were factors influencing seal numbers. The within-day variation in seal numbers along the tidal cycle was also investigated through the use of correction factors which revealed that counting-surveys should be performed around low tide when corrected estimates have a small uncertainty. The unexpected between-days variation in seal numbers, together with the investigated quality of aerial surveys, revealed the need for replicate counts at haul-out sites to provide a measure of uncertainty in the population estimates of Norwegian harbour seals.
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