What advancement allowed for the German u-boats to become less of a threat? WW2

The use and improvement of radar technology was one of the most important proponents in the fight against submarines. Locating submarines was the first step in being able to defend against and destroy them ... From 1943-1945 radar equipped aircraft would account for the bulk of Allied kills against U-Boats.

Floating Coffins. During 1944 the U-Boats sank 120 merchant ships, at the incomprehensible cost of 130 submarines. The casualty rate among the submarine crews was 75%: three of every four sailors who went to sea did not return.