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An ailing 12-year-old dolphin that was part of SeaWorld's stable of underwater mammals died last week — becoming the fourth death since last summer, said park officials in San Antonio.

A necropsy is being done to determine how Dart died, SeaWorld said in a blog post. The male Pacific white-sided dolphin had been in the park's care since 2003.

"Dart had been monitored for health-related issues for about a month and was under constant veterinary care," said SeaWorld, adding that he was with his two dolphin companions, Betty and Hailey, and caretakers when he died.

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Betty, at age 37, is the oldest Pacific white-sided dolphin at SeaWorld San Antonio. The species can live for over 40 years, according to the National Oceanic and Atmospheric Administration.

"She is currently under 24-hour veterinary care for indications of inflammation or a possible infection," SeaWorld said.

Dart's death comes after a series of animal deaths at the park: a newborn beluga died last July after being born premature; a 2-year-old beluga whale died in November after suffering gastrointestinal problems; and a killer whale died in December from a bacterial infection, reported the San Antonio Express-News.

PETA, which has called into question the deaths and SeaWorld's practices, said in a statement to the newspaper that the latest incident highlights how the dolphins should be in a proper seaside sanctuary.

Related: After Stock Plummets, SeaWorld Announces New Orca Environment

SeaWorld San Antonio is planning an expansion that will provide a new habitat for its bottlenose dolphins.

Instead, PETA claims, the parks is planning to open "yet another dolphin tank that will sentence more of them to a sad life and possible early death."