A dolphin's miscarriage at the Vancouver Aquarium has renewed complaints from activists who oppose keeping marine mammals in captivity.

Hana, a 12-year-old Pacific white-sided dolphin, miscarried Tuesday for the second time in less than a year.

Hana has been at the Vancouver Aquarium since 2005.
(CBC)

Veterinarians believe the dolphin fetus had been dead for several days when they first noticed something was wrong with Hana.

Annelise Sorg, with the group No Whales in Captivity, said there has been only one successful pregnancy at the aquarium in nine tries.

"Three orca whales, three beluga whales and two dolphin babies have died there, and only one has survived," she said.

"That's a terrible, terrible record and the aquarium should not be trying to breed these animals."

The aquarium's veterinarians don't know why Hana miscarried. If it wasn't due to a bacterial disease, they said, she may simply be unable to carry offspring to term.

Veterinarian Marty Haulena added that everyone at the aquarium is upset over the miscarriage.

Excitement over the pending birth had been building ever since the aquarium announced in January that Hana was pregnant. The calf's birth was expected to take place sometime this summer.