[MorganThePirate]

As all of us know World War 2 submarines had quite limited underwater speed, range, and endurance. They usually sailed on the surface, especially at night, and submerged only when they had to, to avoid being detected and attacked.
Japanese antisubmarine forces consisted mainly of their destroyers, with sonar and depth charges. However, Japanese destroyer design emphasized surface night fighting and torpedo delivery over anti-submarine duties. By the time they finally developed a destroyer escort which was more economical and better suited to convoy protection, it was too late to save their shipping lanes. The Japanese Army also developed two small aircraft carriers and Ka-1 auto gyro aircraft for use in an antisubmarine warfare role.
The Japanese depth charge attacks by its surface forces initially proved fairly unsuccessful against US fleet submarines. Unless caught in shallow water, a US submarine commander could normally escape destruction, sometimes using temperature gradients (thermo clines). Moreover, IJN doctrine emphasized fleet action, not convoy protection, so the best ships and crews went elsewhere. During the first part of the war, the Japanese tended to set their depth charges too shallow, unaware US submarines could dive below 150 feet (45m) :p . Unfortunately, this deficiency was revealed in a June 1943 press conference held by US Congressman Andrew J May and soon enemy depth charges were set to explode as deep as 250 feet (76m) . Vice Admiral Charles Lockwood later estimated May's revelation cost the navy as many as ten submarines and 800 crewmen. Much later in the war, active and passive sonar buoys were developed for aircraft use.
I think also that the Japanese antisubmarine forces used the magnetic anomaly detectors that they were first employed to detect submarines during World War II. Also the US used them.. MAD gear was used either towed by ship or mounted in aircraft to detect shallow submerged submarines :hmm: .
To reduce interference from electrical equipment or metal in the fuselage of the aircraft, the MAD sensor is placed at the end of a boom or a towed aerodynamic device. Even so, the submarine must be very near the aircraft's position and close to the sea surface for detection of the change or anomaly. The detection range is normally related to the distance between the sensor and the submarine. The size of the submarine and its hull composition determine the detection range. MAD devices were usually mounted on airplanes.