One of two baby bottlenose dolphins born at the National Aquarium in April was found dead in its pool Tuesday morning. A necropsy is being performed to determine the cause.

"We don't really know what happened. There was no abnormal behavior. No signs of anything. It was pretty sudden," said aquarium spokeswoman Jen Bloomer. The second calf appeared healthy and was nursing regularly.

Mortality rates are high — about 33 percent — for baby dolphins, whether they're born in the wild or in captivity. Of 14 calves born at the aquarium since 1992, five died within their first year of life. Two more died as juveniles.

Even so, Brent Whitaker, deputy executive director for biological programs at the aquarium, said the death "is a difficult one for all of us who work tirelessly to provide our animals with the absolute best care."

He said the male calf's mother, Maya, was a first-time mother, and "we had initial concerns about the amount of time it took for the calf to nurse and receive the antibody-rich milk needed to fight off infection."

Even so, he said, "the calf appeared to be strong and was showing no signs of abnormal behavior patterns. Just this week he was active and playing with our other young calf."

Bloomer said the calves were being watched in 18-hour shifts. The staff left at midnight, and the morning shift found him dead at 6 a.m.

The necropsy — the animal equivalent of an autopsy — is being performed at the Johns Hopkins Comparative Pathology Lab by aquarium and Hopkins personnel. Results are expected in about a week.

frank.roylance@baltsun.com

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Maryland weather blog: Frank Roylance on meteorology

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