western gray whales were tagged seven years ago to monitor migration .
the then nine-year-old female called varvara swam from russia to breeding grounds near mexico during five-and-a-half months in 2011 .
gray whales typically do n't feed during migration , which has led researchers to believe she did n't eat during the long journey .
previous record was held by a humpback who swam 11,706 miles in 2011 .