A harbor seal pup died at the National Aquarium on Tuesday, but the staff appears to be successfully nursing a second one back to health.

The first seal pup was brought to the aquarium after it washed ashore on an Ocean City beach Jan. 2. The seal had cuts in its flippers, was grossly underweight and was suffering from a respiratory infection, aquarium officials said. They are not certain what made the animal sick.

But a second animal, which beached itself on Dam Neck Beach in Virginia, has gained 9 pounds since her arrival on Dec. 29 and appears to be recovering.

The aquarium returns animals to the ocean or bay if they recover.