The First World War at sea is often defined by two events: The Battle of Jutland and the Unrestricted Submarine Campaign waged by the Germans (First in 1915 and then again in 1917 to 1918). These key moments often entirely overshadow the rest of the war at sea, and the war at sea outside of the North Sea. In this case they have overshadowed Austria-Hungary’s usage of Submarines!

Your question brings up the Italian, or better to say, Entente blockade of the Adriatic. This was known as the Otranto Barrage and was primarily designed to stop Austro-Hungarian and German U-Boats from escaping the Adriatic. It was operated by British, Italian, and French vessels who deployed nets (they were towed from “drifter” vessels) from the Otranto to Fano island. The idea was for the net barrage to be so wide that the U-Boats had no choice but to dive and deplete their batteries – forcing them to surface in the barrage where they would be attacked and sunk or captured. They would be assaulted by a variety of British, French, and Italian submarines and axillary vessels. In 1918 American “subchasers” even joined the barrage.

But for all that effort the barrage was fairly porous. The waters were deep enough that U-Boats could dive under the nets, and due to the size of the straights, were able to find gaps in it. It did not stop many U-Boats. Thus Austro-Hungarian submarines had a pretty free hand in the Mediterranean where the war against them took on other forms, primarily in anti-U-Boat patrols, and later through convoying.

The first Entente Antisubmarine Warfare (ASW) defenses took the form of “roving ASW patrols” with patrols being assigned to a “zone”. The number of zones did change over the course of the war, but is not the primary reason for the easy time that the Austro-Hungarians (and Germans) had in the Mediterranean. The bigger disconnect comes from the lack of unity and methods of ASW. The French and British, for example, used completely different methods in their respective zones. The British preferred to have their vessels dispersed over a wide area of a patrol route, while the French were insistent on their vessels following a patrol route closely. When an escorted merchant vessel (often these were important vessels such as troop transports) would reach a zone controlled by another nation, the escorted vessel was supposed to be handed off to another escort. In many cases the follow-up escort just never showed and left vessels undefended and easy prey for a U-Boat. They couldn't agree and find a system that effectively worked for coalition war.

Convoying wasn’t introduced until September of 1917 and it wasn’t as successful as in the Atlantic. The biggest reason was that the Mediterranean itself was a warzone (unlike the North Sea/Atlantic where only part of it was considered a War Zone) meaning that convoys had to be escorted all the time, and that the conditions were much more favorable to U-Boat attacks. In addition, it seems that the Entente forces were a bit less aggressive in utilizing weapons such as depth charges, which in the North Sea/Atlantic were a killer.

That said, depth charges were still the number one weapon of the Entente against the U-Boats in the Mediterranean. Gunfire-Armed merchant ships sunk no U-Boats in the Mediterranean, decoy vessels sunk none, only two U-Boats were rammed, and mines played a much smaller role. Italian, French, and British submarines were used in the war against the U-Boats and did sink some! Although again, these were in relatively small numbers. Austro-Hungary lost only about 8 submarines in the war. However, their fleet was bolstered by the presence of German submarines operated by German crews. The Submarine commander who sunk the most tonnage ever was Lothar von Arnault de la Perière. He was a member of the German Navy and was transferred to the Mediterranean.

So in short, the disunity of the Entente, combined with the conditions of the Mediterranean itself, made it that U-Boats were extremely effective against Merchant shipping. In 1916 for example, 36% of all Entente shipping sunk worldwide was sunk in the Mediterranean.