carrier as part of Operation Spotter. A further run by Eagle delivered nine Spitfires. The Club Run (delivery of aircraft to Malta by carrier) became more frequent through 1942. Then, despatched 47 more aircraft (Operation Calendar) on 13 April 1942. All but one reached the island. While the Spitfires were a match for the Axis aircraft, many of those delivered in March and April were destroyed on the ground and in the air, where they were outnumbered; for five days in April there was just one Spitfire available to defend the island, for two days there were none. The Germans had watched their delivery and pressed home heavy attacks. By 21 April 1942 just 27 Spitfires were still airworthy, and by evening that had fallen to 17. The overwhelming Axis bombardments had also substantially eroded Malta's offensive naval and air capabilities. By March–April 1942, it was clear the Luftwaffe had achieved a measure of air superiority. The Regia Aeronautica also pressed home attacks with determination. Often, three to five Italian bombers would fly very low over their targets and drop their bombs with precision, regardless of the RAF attacks and ground fire. Along with the advantage in the air, the Germans soon discovered that British submarines were operating from Manoel Island, not Grand Harbour, and exploited their air superiority to eliminate the threat. The base came under attack, the vessels had to spend most of their time submerged, and the surrounding residences where crews had enjoyed brief rest periods were abandoned. Mine-laying by Axis aircraft also caused a steady rise in submarine losses. By the end of March 1942, 19 submarines had been lost. The effectiveness of the air attacks against Allied naval assets was apparent in the Italian naval records. In April, 150,389 tons of supplies that were sent to North Africa from Italy reached their destination out of a total of 150,578. Hitler's strategy of neutralising Malta by siege seemed to be working. Kesselring reported to the German High Command that "There is nothing left to bomb." The determination of the Axis effort against Malta is indicated in the sorties flown. Between 20 March and 28 April 1942, the Germans flew 11,819 sorties against the island and dropped 6,557 tons of bombs (3,150 tons on Valletta). The Germans lost 173 aircraft in the operations. The Allies moved to increase the number of Spitfires on the island. On 9