Instinctive behavior occurs when an animal has a particular internal state while it is in the presence of a specific external stimulation called a releaser or a sign stimulus.
How many ships did the U-boats sink? During the war the U-boats sank about 2,779 ships for a total of 14.1 million tons GRT. This figure is roughly 70% of all allied shipping losses in all theatres of the war and to all hostile action. The most successful year was 1942 when over 6 million tons of shipping were sunk in the Atlantic.
The pendulum swung dramatically from the early days of rich pickings for U boats to a killing ground as technology, convoy tactics, air cover and, of course, intelligence (Enigma) improved. Also, if the U boat figures are amazing, imagine the fleet of 5,000+ ships they sank and the 75-80, 000 sailors who died.