The loss of another dolphin at the Minnesota Zoo -- the sixth at the zoo since 2006 -- has painfully prodded the staff to ask whether they could be doing anything differently to help the popular creatures thrive.

Taijah, an Atlantic bottlenose dolphin born at the zoo in 2010, died Monday night of complications from a stomach ulcer, the zoo said Tuesday. Another dolphin died last February and one was stillborn in 2009. Three others died in 2006.

After each death, zoo officials say they've combed for clues, looking for a pattern that might suggest something the zoo could change in how the animals are housed or cared for, but Kevin Willis, the zoo's director of biological programs, said he has not seen a pattern.

"It's not like they are all having respiratory infections and you could say there is something wrong with the buildings," Willis said.

"We are soul searching about whether we are responsible stewards of the animals we care for, asking, 'Is there anything that connects these deaths? Is there something we are doing wrong?'"

"The Minnesota Zoo has obviously been struggling," Willis said.

"We have had deaths of young animals that are completely unexpected."

An autopsy report on Taijah found an ulcer in the dolphin's stomach, around which a clot had formed. It was blocking correct passage of food but it should not have killed her, Willis said.

That means the zoo will look further on Wednesday for a fungal or bacterial infection that might have caused the ulcer.

The zoo, in Apple Valley, now has two dolphins, Taijah's parents: mother Allie, 24, and father Semo, who at 46 is one of the oldest dolphins in human care.

Willis said Semo and Allie are aware that Taijah is no longer with them but might not have comprehended that their calf has died.

"When we removed the body from the pool, the mother came over and was popping up and looking at her," he said.

Dolphin presentations on Tuesday at the zoo amphitheater were canceled. The zoo hopes to resume them on Wednesday.

The zoo displays the animals in a large pool that is flanked by two smaller pools, all connected underwater. At regular times, zoo visitors can see the creatures moving through training exercises.

Because of Semo's age and because Allie had been still nursing her calf, they have not been performing high jumps.

The dolphin pool has been in the news because Gov. Mark Dayton included $7 million for its repair in his state building proposal.

Willis said the pool needs a new epoxy coating to keep the concrete waterproof and to protect the structure from damage by saltwater.

The condition of the tank has no connection to the death of the dolphin, he said.

A difficult few years

An older dolphin, April, age 44, died last February. A dolphin was stillborn at the zoo in 2009 after what had been a successful pregnancy, Willis said.

Three others died in 2006: Rio, age 35, along with his son, Harley, a baby dolphin that died in 2006 after running into the wall between the pools. "We don't know why, we don't understand that one," Willis said.

Another dolphin, Ayla, 14, was euthanized in 2006 because she was in pain from a spine defect.

April's daughter, Spree, 8, was moved to another zoo after her mother died so that she could socialize with other females, Willis said.

National experts found nothing to criticize in the zoo's handling of the animals.

"Of the six animals that died, two were animals that lived well over the median life expectancy of dolphins in the wild. One was stillborn," said Marilee Menard, executive director of the Alliance of Marine Mammal Parks and Aquariums.

Most dolphins seen at parks and zoos are breeding and reproducing under human care and few are captured from the wild, she said.

There is no evidence they are under stress, she said.

Captive "dolphins live almost twice as long as those in the wild."

The Association of Zoos and Aquariums in Silver Springs, Md., which gives the zoo its accreditation, has found that the Minnesota Zoo maintains the required high standards for animal care, said association spokesman Steve Feldman. "We are constantly monitoring that they are meeting those high standards."

Laurie Blake • 952-746-3287 Star Tribune staff writer Paul Walsh contributed to this report.