Dönitz's crews faced danger from the outset. The transit routes through the Bay of Biscay were heavily patrolled by aircraft. From May to December 1943, 25 U-boats were sunk by Coastal Command, more were sunk by the USAAF and Royal Navy—five and four respectively; with one shared by the navy and Coastal Command. To counter radar aircraft, Dönitz ordered his submarines to group together and merge their powerful anti-aircraft armament together while surfaced and recharging their batteries, after initially ordering the groups to remain surfaced throughout the journey and fight off aerial attackers with gunfire. The decision was to cost BdU heavy casualties. A group of U-boats were more likely to attract a radar contact, and Allied pilots soon learned to swarm their targets. Dönitz ordered his captains to traverse the Bay under the lee of the neutral Spanish coast, with a sharply rising coast which shielded U-boats from radar. After 4 August 1943, the number of destroyed U-boats fell from one every four days, to one every 27 until June 1944.