PLYMOUTH (CBS) – A female right whale and her calf were spotted off the coast of Plymouth on Saturday.

An aerial survey conducted by the Provincetown Center for Coastal Studies confirmed the presence of the whales, which comes on the heels of an unusually early right whale sighting in December.

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Right whales usually do not arrive in Cape Cod Bay until April.

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Researchers at the New England Aquarium have encountered this mother whale before, and identified her as Wart, a whale who faced a life-threatening entanglement for several years.

In 2010, researchers were able to free her from the entanglement after six attempts over three years. But after not being sighted for some time, there was concern she may not have survived.

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There are estimated to be only 500 North Atlantic right whales remaining, and the continued survival of Wart and her ability to reproduce mark positive signs for the population of the species.