The U-boat fleet's successes in 1940 and early 1941 were spearheaded by a small number of highly trained and experienced pre-war commanders. Otto Kretschmer, Joachim Schepke, and Günther Prien were the most famous, but others included Hans Jenisch, Victor Oehrn, Engelbert Endrass, Herbert Schultze and Hans-Rudolf Rösing. Although skilled and with impeccable judgement, the shipping lanes they descended upon were poorly defended. The U-boat force did not escape unscathed. Within the space of several days in March 1941, Prien and Schepke were dead and Kretschmer was a prisoner. All of them fell in battle with a convoy system. The number of boats in the Atlantic remained low. Six fewer existed in May 1940 than in September 1939. In January 1941 there were just six on station in the Atlantic—the lowest during the war, while still suffering from unreliable torpedoes. Dönitz insisted that operations continue while "the smallest prospect of hits" remained.