BOSTON, Nov 9 (Reuters) - A dolphin calf was in critical condition on Thursday at the National Aquarium in Baltimore after its mother died during an attempt to move the animals from a river to the open ocean, the National Oceanic and Atmospheric Administration (NOAA) said.

About 30 people from 10 federal, state, local and private organizations helped in the capture and attempted transfer from the Shrewsbury River to the Atlantic off Monmouth Beach, N.J. The operation took place on Wednesday.

Early evidence from an examination showed the mother dolphin, a bottlenose, was suffering from an infection in its bloodstream, lung problems and a thinned coat of blubber, officials said.

While the dolphin was in bad shape, it may have been the capture that killed her, said Teri Frady, a NOAA spokeswoman in Massachusetts.

"The capture put stress on her and that was part of the problem," Frady said, noting that the cause of death was not yet established.

Bottlenose dolphins are not endangered, but each year officials respond to about 100 strandings around the country, Frady said.

She had no figures for the total cost of the operation, but said that typically spending by private organizations far outweighed that by government agencies in this type of rescue and transfer.

It was not yet clear what was wrong with the dolphin calf, which has not been named, said Dawn Jennings of the Baltimore aquarium. She said tests were being conducted to determine what, if anything, could be done to help.

The dolphins had been under surveillance since September, when they swam into the river. Dolphins often enter the river in the autumn, but as the weather turned colder and the dolphins showed no signs of heading south to warmer waters, officials decided to intervene.