Debunking common assumptions about the fidelity of the northern elephant seal to breeding and molting sites

Northern elephant seals spend approximately eight months foraging at sea and only return to land for about four months to breed and molt.  Researchers have long assumed that these seals exhibit fidelity towards specific sites, returning annually to both molt and breed in the same location.  However, recent research calls this assumption into question; from 2004 to 2010, 5.3% of 297 foraging trips by satellite-tagged seals terminated in different colonies.  To explain this discrepancy, our project evaluates a long-term mark-resight dataset on flipper-tagged seals in the colony at Año Nuevo state reserve, focusing on seals that have also been tracked with satellite tags.  We will use these data to discriminate between the animals seen at Año Nuevo during the breeding season, molting season, and both.  This will allow us to categorize the animals into two groups: those that came to Año to molt, and those that came to breed only.  To determine whether molting and breeding in separate locations is ecologically advantageous and a possible motivator behind the seals’ unexpected behavior, we will analyze collected data on the mass gained by the satellite-tagged seals during foraging trips.  We will also consider the track lengths of satellite-tagged seals, to discern whether migrating to different sites to breed and molt affected the total length of their migration.  Our project not only has the potential to explain population dispersion, the formation of new colonies, and continued gene flow between populations; it also has the potential to affect the way researchers view mark-resight data.