From ‘Hitler's U-Boat War: The Hunted, 1942-1945’ by Clay Blair, p356Many historians of the Battle of the Atlantic write that the Allies, acting on naval Enigma decrypts, wiped out the U-boat tanker force in the summer of 1943, thereby crippling the operations of the Type VIIs. This is a myth. The Allies did indeed virtually wipe out the U-tanker force that summer, sinking nine of twelve boats at the front (including XB minelayers). However, according to a recently declassified top-secret postwar American study, only two of the nine U-tankers were sunk with the help of Enigma decrypts. Most boats were sunk or forced to abort by Commonwealth and American aircraft engaged in the intense Bay of Biscay anti- U-boat campaign (Derange/Musketry/Seaslug) and by American "jeep" carriers, mostly as a result of ill-advised personal orders from Karl Donitz for all U-boats to remain surfaced and fight back if detected and attacked by aircraft.
Blair was referring to Special Research History SRH-368 ‘Evaluation of the Role of Decryption Intelligence in the Operational Phase of the Battle of the Atlantic, U.S. Navy OEG Report #68, 1952’.
These are the relevant pages from that study: