Shares of Mesa Air Group Inc. fell 6.3 percent to $2.22 after the regional carrier said Delta Air Lines Inc. wants to end a flight agreement that covers almost one-fifth of Mesa's fleet, according to a report from Bloomberg News Service

Delta intends to terminate a contract under which Mesa's Freedom Airlines unit flies ERJ-145 aircraft for the larger carrier, Mesa said in a statement late yesterday that called the notice "a total surprise."

Mesa, which is the parent company of interisland carrier go!, said it would challenge Delta's assertion that Freedom failed to meet required completion rates.That contract involves 19 percent of Mesa's aircraft and won't be easily replaced, Standard & Poor's equity analyst Jim Corridore said today in a note. He cut Mesa to "sell" from "hold" and lowered his share-price target to $2, from $3.