Northeast Region Stranding Network

What Happens After Release?
(Movements and behavior of rehabilitated seals as determined by satellite linked time depth recorders)

Greg Early 1, Robert Cooper 1, Scott Kraus 1, Michael Williamson 2
1. New England Aquarium, Central Wharf, Boston, Massachusetts 02110
2.Wheelock College, 200 The Riverway, Boston, MA 02215


Little is known about the behavior and movements of rehabilitated marine mammals following their release into the wild. Although the New England Aquarium has, over the past twenty years, tagged and released over four hundred seals and cetaceans, less than five percent of these animals have been re-sighted. Most of the tagged animals that are reported are reported dead.

Over the past two years we have tagged eleven seals of three species (hooded, harbor and gray seals), rehabilitated and released by stranding facilities in the Northeast Region (New England Aquarium, Mystic Aquarium and the Marine Mammal Stranding Center in Brigantine). The seals were tagged with satellite linked time depth recorders (STDR), programmed to provide data on dive depth, dive duration, time at depth and haul out activity. Locations were determined by, and data was relayed to earth orbiting ARGOS satellites. Locations were posted daily to an educational web site (WHALENET at http://whale.wheelock.edu)

The seals ranged in age and size from an adult hooded seal (600 lbs) to young-of -the-year harbor seals (80 lbs). Tracks have lasted from 25 to 275 days›(four tags are still currently being tracked).› Preliminary data indicate that three of the tagged seals most likely did not survive.›Unlike survivors, non-survivors (average track 30 days) do not appear to demonstrate localized movements following release.› Dive patterns appear less complex with predominantly shallow, short dives.

The different species have shown a wide range of scale of movements--from local movements of several hundred miles (harbor seals), to mid-range movements of several thousand miles (gray seals), to large- scale movements tens of thousands of miles (hooded seals).› Similarly, dive behavior differs greatly between species.

Following release, most seals leave the release area and spend little time near or on shore, perhaps, in part accounting for the low level of resighting success from conventional, visual tags or marks. This study demonstrates that despite the limitations of satellite tracking (cost and accuracy of locations) this technique can provide definitive answers to the question of the success of rehabilitation efforts.