Chapter 8: Release

“It was a giant, wondrous new world for 501. For all but the first three days of her life, she had lived in tanks. Now there were great expanses of water contained by riverbanks, deep channels, and coastlines, not walls made of glass, metal, and concrete.”

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By December 14th, 501’s growth rate had already begun to slow down and, proficient as she was with crabs, diving, and social maneuvers, her behavior was becoming indistinguishable from the adults around her. 501 had reached her final milestone as a pup: weaning.

Once she was weaned and away from Toola’s watchful eyes, her next destination would hopefully be the wild.  Before the stress of separation, 501 was given final preparations. A radio transmitter was surgically implanted in her abdomen and flipper tags were attached to her rear flippers. The transmitter will give SORAC a way to find her, even in the fog, and flipper tags will be essential for the team to identify her from a safe distance. The radio transmitter might last 3 years and the tags may be the only way trackers will know who she is.

See 501's transmitter surgery and flipper tags attached

On the 11th of January 2011, after an incredibly smooth dependency period with her unlikely otter caretaker, Otter 501 was weaned from Toola. Their bond had grown strong. They had shared food, idle grooming sessions, and naps in the sun, as any biological mom and pup pair. After all that, this separation was absolute. The two would never see one another again.

Typically, pups are weaned somewhere between 24 and 28 weeks. But for 501, because of tight caretaker logistics, it happened at 31 weeks. Even with nearly a month extra at Toola’s side, the separation was not easy. 

In the wild, the mother dictates when weaning will occur and, much like with Toola and 501, it is an abrupt change. One moment mom might be feeding and grooming her pup, the next moment she cuts off the pup completely and the two might never see one another again.

So, to 501, this sudden estrangement was a surprisingly natural process. In fact, all pups get over this separation rather quickly.

However, for Toola, the story was a little bit different. She didn’t get to decide when her adoptive pup would be taken away.

“It’s not uncommon for the females for a day or two days or even more to be completely out of sorts,” said Karl Mayer, sympathetically. “They’re angry. They’re vocalizing. We have to put them into quarantine for a while until they clear medically to go back onto the exhibit. They’re oftentimes vocal. They won’t eat. They’re not grooming themselves well. So, it’s a much more difficult separation for the mother than it is for the pup in this case.”

501’s weaning had been delayed because the next step in her long return to the wild was a tricky one. If she was going to make it in the wild, caretakers needed to be absolutely sure they had the logistics and manpower required for 501’s release.

501 and Toola are separated forever

After 501 was separated from Toola, she was often kept with other otters. Initially this was a critical move. Animals who have just been weaned do better when they are around other sea otters. This period also gave 501 a chance to adapt to life without a watchful guardian. As caretakers looked on, 501 became a strong, independent female. She was ready for the wild.

Her life was on the verge of the biggest leap yet. Otter 501 was transported to Elkhorn Slough, a large estuary brimming with wildlife midway up the coast of Monterey Bay, on May 8th. At 18.0 kilograms (39.6 pounds), she was a robust weight for a female. Mayer, who performs many of the releases himself, loaded her crate onto a boat, and motored out to a quiet, far back section of the Slough. With little ceremony, Mayer opened 501’s crate door and within seconds she was over the boat’s edge and in the murky water, with only a muffled “plop” and a growing ring of water to note her passing.

Watch as 501 is released into Elkhorn Slough

“Day one observations are frequently the same for these animals, which is that they’re spending their time in an exploratory mode,” said Mayer. “They are doing a lot of swimming and traveling but not necessarily covering large amounts of ground. There’s not a real focus on foraging. They’re probably opportunistically diving and looking along the bottom while they’re moving around and getting the lay of the land.”

Otter 501's new home

It was a giant, wondrous new world for 501. For all but the first three days of her life, she had lived in tanks. Now there were great expanses of water contained by riverbanks, deep channels, and coastlines, not walls made of glass, metal, and concrete.

That dramatic spatial difference commanded 501’s curiosity as she adapted to the novelty and located resources that would help her survive. But this process took time and could overwhelm and obstruct important needs—especially eating.

501 was no longer getting food served to her, and in the wild, otters forage for at least 5 hours a day. Now she had to locate and capture food on her own, in a place she’d never been before. Many animals, and even humans, can survive long durations without eating. This is not the case with sea otters.

If Otter 501 didn’t eat, she couldn’t survive for long. Sea otters have such a high metabolism that she would lose weight very quickly and die.

Capturing sea otters then following them after release isn't easy

After a tense 24 hours, day two did not get better.

Karl Mayer was back out observing 501 early that day. “We’re starting to see some more observations of stress, including pacing and self-suckling. In her case, she was suckling one of her forelegs, her paw,” Mayer said. “Again, day two, we did not see any attempts at foraging, really, at all.”

It wasn’t all bad news. 501 exhibited a little positive behavior, such as intermittent grooming. But this was not enough to offset SORAC’s growing concern about 501’s transition to the wild. Released animals can go downhill quickly, or worse, travel somewhere inaccessible. At the end of day two, they had to start thinking about recapture. An airplane pilot was at the ready to track her in case 501 made a break for the open ocean.

Time lapse of Boy's Club at Elkhorn Slough

Telling signs of 501’s high stress levels in this new world escalated on day three. She stress-groomed, sucked her paw, and swam back and forth over short stretches of water.

As her observers fretted, 501 discovered a back channel of the Slough and ducked out of their view. Without a visual on 501, there was no telling if her situation would continue to deteriorate or if she would discover that the abundant green crabs where she headed were easy to catch.

On day four, without much information to go on, Mayer determined that 501’s chances in the wild were just too slim. It was time to recapture her, before SOARAC’s sliver of opportunity to help ended.

Watch sea otters with a night vision scope

“There have been probably five or six that have been in the realm of the epic Elkhorn Slough recaptures and, certainly, this was one of them,” said Mayer. 

501 hadn’t made it easy on them. She had entered a remote area of Elkhorn Slough with narrow side channels, saturated shoreline, and mucky flats. Mayer and Sandrine Hazan, another sea otter biologist for SORAC, kayaked in and fought through waist- and chest-high mud. Then, posted deeply in the muck, the two scooped at 501 with flimsy nets when she occasionally surfaced in a tiny pool of muddy water. Evidently Toola’s lessons at evading capture had paid off.

“You know, it was like whack-a-mole,” Mayer said, shaking his head. But after several tiresome hours, they got her.

501's evasive maneuvers in the mud

501 weighed 16.3 kilograms (35.9 pounds) after recapture. This was a weight loss of  only 9%, less than what the SORAC staff had expected. So it seemed that 501 did find some food, likely in the shallow side channels where she was eventually captured.

Once she was transported back to the Aquarium, caretakers gave her a full exam. The first things they noticed were several light bite wounds on her nose, likely from a male sea otter she had encountered during her few days in the wild. These injuries were fairly common for sea otters, but indicated that one of her few interactions with another wild otter hadn’t been positive.

Nose biting is usually ascribed to mating behavior, and females that have just mated often end up with raw red noses. But males will bite the noses of other males, too. Strangely enough, it’s part of the social dynamic of the sea otters. Since 501 was not old enough to reproduce, it’s unclear what motivated her wounds.

When blood tests from the exam came back, Dr. Murray also found that 501 had slightly elevated white blood cell counts, indicating that her immune system was responding to some kind of threat. Dr. Murray treated this as a bacterial infection and prescribed medication.

In Less than four days in the wild, 501 had lost nearly 9% of her bodyweight, sustained injuries from another otter, and may have contracted a bacterial infection. Considering this, and the stress behaviors trackers had observed while she was in the wild, Mayer and Hazen had made the right choice to endure the muck and bring 501 back to safety. However, these types of issues are relatively common on the first release attempt among surrogate-reared otters, and 501 would very likely get another shot at making it in the wild.

During the ensuing recovery period, it was clear that 501 did not think of the Aquarium as a comfort zone anymore. She was feisty and fighting hard whenever she was netted for treatment or routine tank cleaning. In fact, this was a heartening sign to her caregivers. 501’s taste of independence, despite the obvious challenges, had given her fight that would be necessary in the wild.

Unfortunately, 501 had to be quarantined for a month to not infect any of the other animals while her weight and health were restored. 

Learn about the special sea otter named Harmonie

By June 22nd, Otter 501 was back at her pre-release weight of 18.1 kilograms (39.8 pounds). Her medical chart now indicated that she was back to normal health. 501 was cleared for another attempt at release.

Her chart also had a notation for caretakers that 501 was a “very aggressive animal.” Her transformation to becoming a wild sea otter was apparently complete.

After a 42-day stint back in captivity, 501 was re-released into Elkhorn Slough, very near her original release location.

She faced a steep learning curve when she was first released out in the wild—like what to eat, where to sleep, and how to interact with everything in a brand new environment, from mud to eelgrass to currents to other animals.

Although there was still much to learn, 501 was going back to a location in Elkhorn Slough that was no longer new to her. Her first day back played out better than the last.

Another attempt to return 501 to the wild

However, “After a productive day one, 501 seemed to take a step backward on day two,” said Mayer. “Behavior was more erratic. Very little productive foraging observed, increased stress, specifically suckling of the left foreleg, intermittent bouts of rapid swimming.” 

Disappointing though it might seem, sometimes sea otters had to be recaptured multiple times.

But on day three, the tracking team found 501 lingering near a section of Elkhorn Slough used by a large group of female sea otters. It was a sign that she was becoming familiar with her resources and perhaps seeking out companions who knew the ropes.

During the 4.5 hours of observation that day, trackers did not see her exhibit stress behavior. She was foraging most of that time, occasionally finding success.

“Primary prey were paw size or slightly larger cancer crabs, which she frequently ate two or three at a time. She ate a fair number of smaller than paw-sized shore crabs,” Mayer said.

The SORAC team was particularly encouraged to see her eat seven to eight two-paw-sized gaper clams. During days four and five, she continued to find success, feasting on cancer crabs, shore crabs, and gaper clams.

Occasionally she showed signs of stress, which most commonly presented as self-suckling her left forelimb during grooming. But in such a situation, stress is to be expected and it clearly wasn’t keeping 501 from eating or, just as importantly, grooming her coat.

See 501's last weight

By day seven, she seemed to have successfully acclimated to the wild. SORAC called off the intensive tracking for 501 and checked in on her only periodically. By day nine, she was observed still occupying the same popular area of the slough and achieving an 85% success rate when she dove for prey. 

And then on day fourteen, after two weeks spent verifying her competence in the wild, Mayer officially cleared Otter 501. She was a wild sea otter now.

See the 501's last history in captivity
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Otter 501′s home is a wild and diverse place. So are your local parks and open spaces. You can enjoy those spaces and contribute to science by recording what lives there. Join an online community of naturalists and contribute your observations. Have a photo of something wild? Add it here!


Map of recent mammal observations courtesy of iNaturalist.

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Special Features

This Page All
  • Trans. Surgery

    Video Clip: Transmitter SurgerySee 501's transmitter surgery and flipper tags attached

  • Separation

    Video Clip: 501 Weaning501 and Toola are separated forever

  • Map: Elkhorn Slough

    Map: Elkhorn Slough, CaliforniaOtter 501's new home

  • 501's 1st Release

    Video Clip: 501′s First ReleaseWatch as 501 is released into Elkhorn Slough

  • Mini Doc: CTT

    Mini Doc: Catching, Tagging, and TrackingCapturing sea otters then following them after release isn't easy

  • Boy's Club

    Vidoe Clip: Boy’s ClubTime lapse of Boy's Club at Elkhorn Slough

  • Night Vision Scope

    Video Clip: Sea Otters at NightWatch sea otters with a night vision scope

  • ITF: 501 Recapture

    In The Field: 501 Recapture501's evasive maneuvers in the mud

  • Mini Doc: Harmonie

    Mini Doc: HarmonieLearn about the special sea otter named Harmonie

  • Second Release

    Video Clip: Second ReleaseAnother attempt to return 501 to the wild

  • Growth Chart 5

    Growth of Otter 501 to 380-days OldSee 501's last weight

Chapter 1: The Ocean is Cold but Mom is Warm

Chapter 2: Less than Ideal Rescue

Chapter 3: Emergency Treatment

Chapter 4: Species in Danger

Chapter 5: No Substitute for Mom’s Cooking

Chapter 6: The Surrogate

Chapter 7: Furry New Friends

Chapter 8: Release

Chapter 9: A Wild Sea Otter

Video Clip: Transmitter Surgery

For Otter 501 to be tracked and identified in the wild, she needed a radio transmitter and flipper tags. Watch the surgery to insert the radio transmitter, and see how her flipper tags were attached. Warning: this video has some graphic content.

Video Clip: 501 Weaning

It was 161 days since Otter 501 was first introduced to Toola. Now it was time for them to be separated forever. Weaning in the wild is dictated by the mother, and the pup adjusts quickly. While it was still very hard on 501, Toola felt worse because she wasn’t letting 501 go. 501 was taken away from her.

Map: Elkhorn Slough, California

View Map: Elkhorn Slough in a larger map

Found on the edge of Monterey Bay, halfway between Santa Cruz and Monterey, Elkhorn Slough is a large estuary and tidal marsh that runs 7 miles inland. Elkhorn Slough is where 501 was set free.

Video Clip: 501′s First Release

501 was rescued when she was only three-days old. She had been in captivity for more than 330 days. Today was the day she was being released into Elkhorn Slough, along the Monterey Bay, to be wild once more.

Mini Doc: Catching, Tagging, and Tracking

The ability to capture, tag, and track sea otters is essential for scientific study. See how sea otter biologists do it.

Vidoe Clip: Boy’s Club

Near the mouth of Elkhorn Slough, a large raft of sea otters congregates. Called the Boy’s Club, it is made of mostly young male sea otters and males too old to defend territories. Watch as this time-lapse video shows dozens of sea otters coming and going.

Video Clip: Sea Otters at Night

With sea lions barking in the background, watch through a night vision scope as sea otters move from a raft outside of Elkhorn Slough and begin to haul-out on land.

In The Field: 501 Recapture

Even though biologists determined that 501 wasn’t likely to survive after her first release, 501 was not willing to come back to captivity without putting up a fight. See how 501 was recaptured.

Mini Doc: Harmonie

Many years ago, a sea otter pup was rehabilitated at the Monterey Bay Aquarium and released back into the wild. Scientists later discovered that she had about 10 pups throughout her life. She lived a long life, to the age of 15. Warning: this video has some graphic content.

Video Clip: Second Release

It was not uncommon for a surrogate-reared sea otter pup to need release and recapture several times to finally make the full transition to being wild. Watch as Karl and Katie release Otter 501 the second—and final—time.

Growth of Otter 501 to 380-days Old

This graph shows Otter 501’s entire growth history, from rescue to second release at 380-days old. Note that about halfway on the graph, her rate of growth started to slow down. This can be seen where the straight line begins to curve off.

Otter 501′s Development to 380-days Old