Determining Survival of Rehabilitated Seals Using Satellite Telemetry

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


Pinnipeds are the most commonly stranded, and most commonly rehabilitated marine mammals. Each year, hundreds of pinnipeds are released following treatment at rehabilitation centers, zoos and aquaria. With improved medical care and husbandry--and limited space for permanent care--increasing numbers of rescued animals are being released. Despite the number of animals released and a requirement by the National Marine Fisheries Service (NMFS) that all released animals be marked and/or tagged, there is little information about the survival or behavior of re-introduced animals.› Since 1980, the New England Aquarium has received less than a 5% re-sighting of over 300 rehabilitated seals it has released with numbered identification tags. Those animals that are reported are generally dead.

During the past 2 years, however, we have used satellite linked time depth recorders (STDRs) to monitor the movements and behavior of eleven rehabilitated seals of three species (Hooded seal, Cystophora cristata [4], Harbor seal, Phoca vitulina [5], and Gray seal, Halichoerus grypus [2].› The seals ranged in age and size from an adult hooded seal (600 lbs) to young-of -the-year harbor seals (80 lbs). Data and locations were received through Service ARGOS.› The tags were programmed to transmit dive depth, duration and time at depth. 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.