meaning of a part in a sentence

What is the meaning of "placed in a tank" in this sentence?
A small female dolphin was captured by means of a hook placed in a tank.

Does it mean "which was(had been) placed in a tank" or "and she was placed in a tank?"

This is the whole paragraph. Thanks in advance.

A small female dolphin was captured by means of a hook placed in a tank. The hook injured her, and the shock of capture and of being wounded was so great that she could scarcely keep herself afloat. Dolphins, being mammals, must come up for air and cannot live without it. Therefore glass jars were attached to her to act as floats. It was touch and go whether she would live; she seemed so aimless and dispirited. She merely floated about, and her captors were sure she would die soon unless they could think something up quickly. How about a male companion, they thought? They brought in a male Delphinus. Pauline, as they named the female, showed favorable reactions to the newcomer, making the sounds which are recognizable as indicating acceptance. Despite the shock and pain of her injury, she even made some effort to swim. The bottles were removed. The male dolphin tried to help Pauline to swim, pushing under her and bringing her to the surface. This he did with infinite patience, again and again. Pauline recovered. The two dolphins became inseparable, swimming happily around together, playing games, showing every sign of affection. Two months later Pauline died from an infection of the wound that was caused during her capture. As she died, her companion circled round her constantly, emitting a shrill whistle of distress. When she was dead, he continued his cries, swimming round and round for three days, refusing all food. Then he, too, died. An autopsy showed an ulcerous condition aggravated by his refusal to eat. Peritonitis followed, and proved fatal. His ceaseless lamentations after Pauline’s death were one of the saddest displays ever witnessed in dolphin research.