1944
In 1943, the RCAF had begun to successfully harass U-boat operations in Canadian coastal waters and the RCN had grown in numbers and effectiveness to allow more resources to be dedicated to anti-submarine warfare operations in territorial waters. By early 1944, the shipping lanes in the Gulf of St. Lawrence and St. Lawrence River were re-opened to domestic and war-related convoys operating primarily from Quebec City to Sydney.
Late 1944 saw a resurgence of U-boat activity in the St. Lawrence River and Gulf of St. Lawrence. German submarines were being equipped with the snorkel, a telescopic engine ventilation system that permitted continuous underwater operation without surfacing.
U-1223 entered the Gulf of St. Lawrence undetected in early October and is credited with seriously damaging the River-class frigate HMCS Magog on 14 October and sinking the Canadian freighter SS Fort Thompson on 2 November.
Three weeks later, U-1228 attacked and sank the Flower-class corvette HMCS Shawinigan in the Cabot Strait on November 24.
These two German attacks signified the end of the Battle of the St. Lawrence. In May 1945, following Germany's surrender, U-889 and U-190 surrendered to the RCN at Shelburne, Nova Scotia and Bay Bulls, Newfoundland respectively.
After the war, it was shown that the mingling of fresh and salt waters in the region (the world's largest estuary), plus temperature variations and sea ice, disrupted RCN anti-submarine operations and reduced the effectiveness of shipboard sonar systems that were designed to detect submarines. Fog and other weather conditions in the St. Lawrence River and Gulf of St. Lawrence also conspired to hamper RCAF patrols.
Read more about this topic: Battle Of The St. Lawrence