What was the naval tactic used to reduce the sinking of ships by German submarines?

The primary tactic employed by the Allies in WWII to try and initially stem shipping losses to U-boats was the Convoy System. Ships in convoys were easier to screen and protect from attacks, and the chances of more material making it to its destination improved even if there were some losses.

Over the course of the war, the escorted convoy system, decoy ships (armed vessels disguised as merchants), recoverable ASW aircraft, and eventually long range bombers spelled the end of the successes the Germans enjoyed early in the war.

The capture of U-110 by the HMS Bulldog, resulting in one of the biggest Intel coups for the Allies when her Enigma code machine and books were recovered intact, also significantly aided the allied convoy system. Early in the war, the Allies could not break the Enigma code and it was a big factor in heavy shipping losses. Afterward, they were able to

read enough code to help convoys avoid known U-boat patrol locations. At the end of the war, combined efforts led to U-boat losses that were huge - a 75% casualty rate.