Variability in harbour seal counts during consecutive aerial surveys

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European legislation requires the status of harbour seals (Phoca vitulina) to be monitored. Counting methods are designed to reduce the potential effects that date, time of day, tide and weather might have on population estimates. The Sea Mammal Research Unit (SMRU) surveys harbour seals from the air. For consistency, these counts are only carried out during a three-week survey window within the annual moult when, in most areas, greatest numbers of seals are hauled ashore (Thompson & Harwood, 1990; Boveng et al., 2003; Harris, Lelli & Gupta, 2003). In order for long-term trends to be determined from these counts, it is assumed that numbers of seals reach a plateau during this survey window and that the timing of this plateau does not vary with location or between years.