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updated 11/15/2011 10:19:08 AM ET 2011-11-15T15:19:08

After 55 days on display at the Monterey Bay Aquarium, a great white shark was set free into the Pacific a few weeks ago, equipped with both high hopes for its survival and a tracking device to see where it would end up. Within hours, the 52-pound fish died.

The aquarium decided to let the shark go after watching him repeatedly rub his nose against the glass of his 1.5-million gallon tank. Eventually, they knew he'd end up with an infection and they wanted to let him out while there was still time.

The death, which is still under investigation by aquarium veterinarians, was unexpected but not unprecedented. Every year, thousands of marine animals are picked up for research, rehabilitation or exhibition. Some stay in aquariums or zoos for the rest of their lives, but many are carefully returned to the wild.

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And while these releases are often successful -- and getting more so -- the ocean is far more vast and unpredictable than the safe haven of an aquarium tank. There are no guarantees.


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"The ocean is definitely a big place and things go wrong," said Monterey Bay Aquarium veterinarian Mike Murray, who is still fixated on analyzing the mystery of the great white's demise. "I have been thinking about it and browbeating myself and going through everything we have done, wanting to have an answer to what occurred. And I can't come up with a real answer."

Despite longstanding arguments about the ethics of zoos and aquariums, captivity is most often the safest and most luxurious option for animals. They get plenty to eat. They have access to excellent medical care. And they can swim free of threats from predators.

Still, only about 20 percent of marine animals in public facilities actually move in from the wild. And many of those only stay because they are unlikely to survive in the sea.

Meanwhile, thousands of seals, dolphins, manatees and other animals spend weeks to months in zoos, aquariums and rehabilitation facilities, often after they've been rescued after injuries or illnesses. And it is this transient population that continues to teach scientists what makes a release successful, and what leads to failure.

Before deciding when and where to let an animal go, for example, scientists have to think carefully about an animal's social life and seasonal rhythms, said Chris Dold, a veterinarian at SeaWorld Parks & Entertainment in Orlando, Fla.

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Manatees, for example, are highly social during the winter months as they gather around warm water springs. So, releases often happen during those times when individuals can link up with a group. California sea lions, on the other hand, are fairly solitary creatures so the presence of peers is less important. Tracking devices can help experts learn from their mistakes.

"We use the best information we have, based on work from biologists and specialists in the field, to do as much as we can to prepare for success," said Dold, who said that SeaWorld facilities deal with about 1,000 animals each year, including turtles, manatees, sea lions,seals and an occasional whale or dolphin.

"It certainly is disappointing to hear when something goes wrong associated with a release," he added. "But it is part of the practice. There are certainly cases where a release has not worked out particularly well, and in those cases we work hard to learn as much as we can."

By treating wild animals as medical patients, veterinarians also often add basic knowledge to science about the animals' physiology and the threats they face in the ocean, said Bill Van Bonn, chief veterinarian at the Marine Mammal Center, a non-profit rehab facility in Sausalito that treats hundreds of sick and injured animals each year.

Of the 528 California sea lions, elephant seals and harbor seals that have come to the Center so far this season, he said, more than half have survived to be released alive. As they offer care, scientists collect data about patterns and individual cases to better understand disease outbreaks in the oceans and to try to figure out what causes them.

The case of the recent great white shark's death points out how much there is left to learn about the animals that rule the seas. Everything suggested that the release should go well. But a few days after the shark swam off, his tracking tag sent out a signal that the animal had remained at a single depth for more than 96 hours.

As programmed, the tag floated off and the carcass was never recovered.

© 2012 Discovery Channel

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