Abstract:
During three summer surveys at Prince Edward Island (PEI), southern Indian Ocean (2001, 2004 and
2008), 416 southern elephant seals Mirounga leonina were inspected for identification tags. In all, 42
seals that had been tagged as weaned pups at their natal site were found on Marion Island (MI), 38 of
which could be individually identified by resighting their tag numbers. The majority of the MI-tagged
seals were yearlings or subadults, and all but one were hauled out at PEI for the annual moult. The
attendance rate of the known individuals at their natal island during the annual moult was only 40%,
based on their resighting histories. This was significantly lower than the 77 ± 6% moult attendance
rate estimated for a random MI population sample drawn from the same cohorts (based on 10 000
replications). Annual resight probabilities (considering all haulout phases) was 58% per annum for the
MI seals seen at PEI, and 80 ± 4% for the simulation. Seasonal and annual absences of seals from MI
violate the ‘homogeneity of capture’ assumption of mark–recapture models. When multiple sightings
during any year are treated as a single sighting, resights during other haulouts (e.g. breeding) compensate
only partially for absences during the moult. Therefore, mark–recapture studies undertaken in
archipelagos should ideally include both marking and resighting of individuals on all islands which will
allow discrimination between mortality and local migration.
