Do rehabilitated harbor seals behave differently than wild seals of the same age?

A recent tracking study published by Samantha Sangster and Dr. Martin Haulena of the Ocean Wise Marine Mammal Rescue Centre and Co-authored by SeaDoc Society Science Director Joe Gaydos, aimed to answer that question, shedding new information on the post-release outcome of rehabilitated harbor seals.

The study, titled “Interannual differences in postrelease movements of rehabilitated harbor seal pups (Phoca vitulina richardii) in the Salish Sea” and published in the June 2020 issue of Marine Mammal Science, built upon a study published by the SeaDoc Society in 2012 that looked at one year of wild and rehabilitated harbor seal weaned pup movements. The new study compares rehabilitated harbor seal movement over multiple years, telling a more nuanced story.

The team used lightweight satellite transmitters (pictured here) that were programmed to allow for at least 300 days of transmission.

Few studies have been conducted on postrelease movement of rehabilitated harbor seals pups, even though the species can be found all over the world. Sanger’s study looked at 24 rehabilitated pups: Ten from 2010, five from 2012, five from 2013, and four from 2014. Additionally, 10 wild seals were tagged and tracked in 2010 for comparison.

The maximum linear distance traveled from the release site was similar among the rehabilitated seal groups. Compared to ten wild seals, the 24 rehabilitated pups traveled significantly farther daily, and further over time, than did wild weaned pups, as you can see in the following chart. This was consistent with Gaydos’ previous study.

What was new was that when evaluated over multiple years of release, the duration of transmission was similar between the wild weaned pups and the rehabilitated pups.

This work suggests that rehabilitated harbor seals do not closely mimic the movements of weaned wild pups, particularly during the first few months postrelease, but they do successfully transition to free-ranging animals and can survive beyond our ability to monitor them using traditional telemetry techniques.

Given the time and cost invested in rehabilitating harbor seals, it’s important to track animals postrelease to determine if rehabilitation is offering them the best chance of survival.