WELFLEET (CBS) – Marine Biologists say a pod of North Atlantic right whales has returned to the waters off the coast of Cape Cod this week.

On Wednesday, staffers at the Provincetown Center for Coastal Studies spotted two of the rare whales near Wellfleet.

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Cape Cod Bay is rich in zooplankton, which is right whales’ main source of food. The whales typically show up in Cape Cod waters to feed from mid-January to May.

Last year, 315 North Atlantic right whales were spotted in and around Cape Cod Bay. That’s more than 60% of the species’ entire population.

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According to scientists, rising sea temperatures are allowing zooplankton to thrive in the area, which may be the reason that right whales have begun returning to the area about a month earlier than normal.

Ocean temperatures in 2011 averaged about three degrees warmer than average.

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Right whales are protected under the Marine Mammal Protection Act and the Endangered Species Act. It is illegal to approach within 500 yards of a right whale.