Range-Wide Patterns in Hawaiian Monk Seal Movements Among Islands and Atolls
February 16, 2015
It was once assumed that Hawaiian monk seals did not travel between the Northwestern Hawaiian Islands and the main Hawaiian Islands, but research now shows that in rare cases they do, and even make it down to Johnston Atoll.
Most Hawaiian monk seals reside in the Northwestern Hawaiian Islands where population abundance is declining. A small and growing number also occur in the main Hawaiian Islands, and rare sightings have been documented at Johnston Atoll, south of the Hawaiian Archipelago. Until recently, it was thought that there was essentially no movement of seals between the NWHI and MHI. Here, we present a new analysis of movements throughout the species range based on sightings of tagged seals over a 30-year period. The proportion of seals observed away from their natal sites varied with location but increased until adulthood, when 14% of seals were at non-natal sites. Females at non-natal sites accounted for 10% of all births. No sex differences were found. Annual movements declined rapidly with distance, with very little interchange between locations separated by over 400 km. The highest movement was observed among the relatively closely grouped MHI. In general, the likelihood of seals moving between sites in any given year increased with age class. While relatively rarely observed, at least 10 seals made 14 trips between the NWHI, the MHI, and Johnston Atoll. Due to incomplete and uneven sampling, actual movement probabilities were not estimated. However, the observed age- and distance-related movement patterns, along with confirmation that monk seals freely disperse throughout the Hawaiian Archipelago, will help inform measures to manage and conserve this critically endangered species.