What whale made the longest migration of any mammal on the planet?

Currently, researchers have documented a female Pacific gray whale from a small group of other gray whales that has completed the longest migration on record for a mammal. A nine year old gray whale, named Varvara, swam from Sakhalin island, Russia, to Cabo San Lucas, Baja California, and back beginning in November 2011. She logged 14,000 miles during her 172 day trip (researchers reported the actions of the whale online in April 2015 in the journal Biology Letters).

Until the gray whale, the title of the longest migrating mammal belonged to the humpback whale (Megaptera novaeangliae), which migrates up to 10,190 miles round trip as it travels between its breeding grounds near the equator and the food rich waters of the Arctic and Antarctic as stated in Guinness World Records.