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GALVESTON — Four Gulf sharks died in the Caribbean exhibit at Moody Gardens Aquarium after the ozone level in the water spiked overnight. The elevated levels of ozone were not enough to trigger the tank’s alarm, officials said.

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Moody Gardens in Galveston

Moody Gardens staff members arriving to work early Monday morning discovered two sharks dead and two others barely moving at the bottom of the exhibit, unusual behavior for the fast-moving, aggressive hunters, said Jerri Hamachek, marketing and public relations manager.

Divers tried for almost an hour to revive the two blacktip sharks by injecting them with steroids, but it was too late, Hamachek said.

“It all happened so quickly,” she said. “Our staff is just devastated.”

Other exhibit animals, including sand tiger sharks, brown sharks, sea turtles, eels and tropical fish, were not affected. Hamachek said the sharks, first introduced to the exhibit two years ago, were especially sensitive to ozone changes.

“This exhibit has been open for nine years, and we’ve never had this issue before,” she said. “Obviously, the care of the animals is of the utmost importance. We’re looking at this closely.”

Staff members aren’t sure why ozone levels rose in the exhibit. Ozone, an oxidizing agent, is pumped into the tank to kill bacteria and parasites and to clarify the water. Ozone levels are monitored by a computer, which triggers an alarm if they change drastically.

The alarm never sounded because the ozone levels in the tank never reached a critical point, Hamachek said.

Staff members corrected the levels and lowered the threshold to trigger the alarm earlier in the future, she said.

“We’re not concerned this is going to happen again,” Hamachek said.

The sharks will undergo necropsies so biologists can determine the exact cause of death. They’ll also check the sharks for low-level parasites or bacteria to make sure the other exhibit animals aren’t in danger. Results could be available in three weeks.

The sharks, collected in the Gulf of Mexico, have lived in the million-gallon exhibit for two years, she said. The blacktip sharks and Atlantic sharpnose shark were 2 years old; the blacknose shark was between 5 and 6 years old.

The sharks, viewed in an underwater tunnel at Moody Gardens Aquarium, were a popular attraction among visitors.

Moody Gardens Aquarium is licensed by the U.S. Department of Agriculture and was recently accredited by the Association of Zoos and Aquariums, the nation’s leading accrediting organization for such establishments.

The association inspected the aquarium’s living environments, nutrition, veterinary program, conservation, research, education programs, safety policies and procedures, security, facilities, guest services and animal management and care.

Fewer than 10 percent of 2,400 animal exhibitors licensed by the agriculture department are accredited by the association, according to association information. The agriculture department’s animal care division inspects licensed agencies at least once a year.

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What Are They?

•Blacktip shark: gray with black-tipped fins; average 5 feet long and 40 pounds; migrate through the Gulf of Mexico, found in both inshore and offshore waters but prefer to stay close to coast lines; live up to 12 years

•Atlantic sharpnose shark: gray with long-pointed snout; average 3-3.5 feet long and 16 pounds; native in the western Atlantic from Canada to the Gulf of Mexico, and in the Atlantic along Brazilian coastline; live up to 10 years

•Blacknose shark: gray with characteristic black marking under nose; average 4 feet and 22 pounds; native to the western Atlantic Ocean from North Carolina to Brazil, including the Caribbean Sea, Bahamas and Gulf of Mexico; live up to 16 years

Source: Marinebio.org and Florida Museum of Natural History