108 ships were sunk in the first 20 days of March, and just 15 in the last 10. The official naval historian wrote, "The collapse of the enemy's offensive, when it came, was so sudden that it took him completely by surprise. We now know that, in fact, a downward trend in the U-boats' recent accomplishments could have forewarned him, but was concealed from him by the exaggerated claims made by their commanders." In April Dönitz lost five crews to Coastal Command's ASV Bay offensive. Encouraged by the isolated successes of anti-aircraft artillery installed on submarines, he ordered crews to stay on the surface and fight it out with the aircraft. The decision caused casualties—four boats were lost in the first week of May alone, and three more by the end.