'Kiwbitz' was a German unsuccessful operation to recover four prisoners of war, all experienced U-boat commanders or officers, from the lower reaches of the St Lawrence river in eastern Canada (6 September 1943).

The Kriegsmarine developed a plan to have Fregattenkapitän Otto Kretschmer, Kapitänleutnant Horst Elfe, Kapitänleutnant Hans Ey and Leutnant Hans Joachim Knebel-Döberitz escape from Camp 30, at Bowmanville in Ontario, and make their way some 870 miles (1400 km) through eastern Canada to northern New Brunswick and a rendezvous with a U-boat off Pointe de Maisonnette, New Brunswick. The plan was developed in 1942 and was to be implemented in September 1943. Knebel-Döberitz was the former adjutant of Grossadmiral Karl Dönitz, commander-in-chief of the Germany navy and still in day-to-day command of the U-boat arm, and he and Kretschmer (a major U-boat 'ace'), were thought to be the primary reason behind the operation. Coded messages were sent by mail through the International Committee of the Red Cross to the German prisoners at Camp 30. However, the messages were intercepted by Canadian military intelligence and the Canadian police, which were screening all prisoner communications. The Canadian authorities did not tip off the prisoners that their plans were detected as the Royal Canadian Navy hoped to exploit the situation to seize a U-boat in Canadian waters, which would have been a major intelligence coup for the Allies.

The military, Royal Canadian Mounted Police and camp guards monitored the German prisoners' endeavours as they dug several tunnels, at least one of which would eventually lead outside the camp boundaries. The Royal Canadian Mounted Police and military guards moved in and seized the prisoners as they attempted to escape, and collapsed the tunnel. One of the prisoners, Korvettenkapitän Wolfgang Heyda, managed to escape over the camp’s wall and elude both search parties and a massive police response to make his way on Canadian National Railways passenger trains from southern Ontario to Pointe de Maisonnette. Heyda reached the location at the appointed time only to be arrested by Royal Canadian Mounted Police and Canadian navy personnel waiting to co-ordinate a surface task force that would attempt to seize the U-boat.

The Canadian navy implemented its 'Pointe Maisonnette' operation to attack and/or seize the U-boat. This involved Canadian army as well as naval personnel on shore at Pointe de Maisonnette lighthouse, where a portable surface radar array was established, along with a task force of several warships centred on the 'Flower' class corvette Rimouski concealed nearby. Korvettenkapitän Rolf Schauenburg’s U-536 arrived off Pointe de Maisonnette at the appointed time in September 1943. The waiting Canadian personnel signalled with a light that the escapees were to have used, but Schauenburg was suspicious, particularly after his hydrophones picked up the sound of the nearby Canadian task group’s propellers. He opted to remain submerged and began to evade the Canadian warships, which searched throughout the night and made unsuccessful depth-charge attacks on U-536.

Despite evading the Canadian trap in Chaleur Bay, U-536 was sunk by depth-charge attack on 20 November before regaining a German port.

The Canadian action with U-536 was one of the engagements constituting the so-called Battle of the St Lawrence between a number of U-boats and Canadian naval and air elements right through the lower part of the St Lawrence river and the entire Gulf of St Lawrence, Strait of Belle Isle and Cabot Strait between May and October 1942, September 1943, and again in October and November 1944. During this time, U-boats sank a number of merchant marine ships and three Canadian warships.

The German navy had made no formal plans to attack merchant shipping in the St Lawrence river and Gulf of St Lawrence, despite its activities off the convoy assembly ports of Halifax and Sydney, Nova Scotia, so the early German efforts in the Battle of the St Lawrence were extemporised and opportunistic. The first attack was by Korvettenkapitän Karl Turmann’s U-553, which torpedoed and sank the 5,364-ton British freighter Nicoya at the mouth of the St Lawrence river several kilometres off Anticosti island on 10 June 1942, followed by the 4,712-ton Dutch freighter Leto in the same vicinity several hours later. U-553 then left the Gulf of St Lawrence to return to its established patrol in the North Atlantic.

Before these sinkings, the Gulf of St Lawrence and the St Lawrence river had been guarded by only four Canadian vessels in the form of one minesweeper, two motor launches and one armed yacht. This was clearly an inadequate force, and the Canadian response to the attacks by U-553 was the deployment of five corvettes. Even with these reinforcements, however the force available to guard the St Lawrence area was still wholly inadequate. The incident clearly indicated that the Canadian navy lacked the resources to deal with the situation, and the political repercussions in Canada led to suggestions that Canadian warships allocated to the Atlantic convoys should be recalled to protect Canadian territorial waters. However, the Canadian navy’s priority remained with the protection of convoys to the UK, USSR and North Africa.

Several British escorts were attached to the Royal Canadian Navy for several months during 1942, with convoys in the St Lawrence River and Gulf of St Lawrence being formed between the Canadian navy facilities at Chaleur II in the city of Quebec, Fort Ramsay in Gaspé, Quebec province, and Protector in Sydney, Nova Scotia. Royal Canadian Air Force aircraft patrolled from operations squadrons based at air bases such as Mont-Joli, Bagotville, Chatham, Mt Pleasant, Charlottetown, Summerside, Debert and Sydney as well as various civilian fields, particularly in the Magdalen islands. Blackouts were strictly enforced and army units were sent out on coastal patrols along roads and railway lines.

On 6 July 1942 Kapitänleutnant Ernst Vogelsang’s U-132 sank three freighters (3,382-ton Greek Anastassios Pateras, 4,312-ton Belgian Hainaut and 2,555-ton British Dinaric) off the Gaspé coast, and then damaged another on 20 July, each time escaping attack by the minesweeper Drummondville. There were further German attacks in August, on harbours in Labrador and Newfoundland and on a convoy in the Strait of Belle Isle.

In August and September, three U-boats made a joint raid on the St Lawrence. Kapitänleutnant Paul Härtwig’s U-517 was notably successful, sinking eight merchant vessels (5,649-ton US Chatham, 3,304-ton US Arlyn, 1,781-ton Canadian Donald Stewart, 1,727-ton Canadian Oakton, 5,729-ton Greek Mt Pindus, 3,286-ton Greek Mt Taygetus, 2,166-ton Norwegian Inger Elisabeth and 2,741-ton Norwegian Saturnus) and the Canadian corvette Charlottetown, and damaged another merchant vessel in a two-week period, escaping attacks by escort vessels each time. Fregattenkapitän Eberhard Hoffmann’s U-165 was less successful in attacking merchant shipping but sank the armed yacht Raccoon. The U-boat was harassed by RCAF patrol aircraft and completed its operation in the Gulf of St Lawrence without making more attacks. The continued attacks caused the St Lawrence River and Gulf of St Lawrence to be closed to all trans-Atlantic shipping, though coastal trade was permitted. In practice, although this embargo strained the rail system to Halifax, it simplified the management of Atlantic convoys. The embargo lasted until a time early in 1944.

On 14 October 1942 the 2,222-ton Newfoundland railway passenger ferry Caribou was torpedoed by Kapitänleutnant Ulrich Gräf’s U-69, in the Cabot Strait between Newfoundland and Nova Scotia, with heavy loss of life. U-69 escaped a counterattack by the minesweeper Grandmere.

In November 1942 Kapitänleutnant Friedrich-Wilhelm Wissmann’s U-518 sank two iron ore freighters and damaged another at Bell island in Conception Bay, Newfoundland, in transit to its patrol area off the Gaspé peninsula where, despite an attack by a Royal Canadian Air Force patrol aeroplane, it landed a spy at New Carlisle, Quebec, although the spy was captured at the New Carlisle railway station shortly after landing on the beach.

U-boat losses during 1942, following the entry of the US Navy into the Battle of the Atlantic and a decline in German shipbuilding capability to replace battle losses, saw the U-boat fleet redeployed to the primary Atlantic convoy routes to disrupt the Allied war resupply effort. This effectively saw the U-boats being withdrawn from the St Lawrence river and Gulf of St Lawrence by the end of the 1942 shipping season. During 1943, Canadian warplanes had begun successful harassment of U-boats in Canadian coastal waters and the Canadian navy had grown in strength and capability, making it possible to allocate greater resources to the dedicated anti-submarine role in Canadian territorial waters.

At a time early in 1944 the shipping lanes in the Gulf of St Lawrence and St Lawrence river were reopened to domestic and war-related convoys operating primarily from the city of Quebec to Sydney. The period late in 1944 saw a resurgence of U-boat activity in the St Lawrence river and Gulf of St Lawrence. The U-boats were increasingly equipped with the Schnorchel, a telescopic engine ventilation system that permitted continuous underwater operation on Diesel engines without the need to surface. Oberleutnant Albert Kneip’s U-1223 entered the Gulf of St Lawrence undetected early in October and is credited with seriously damaging the frigate Magog on 14 October and sinking the 7,134-ton Canadian freighter Fort Thompson on 2 November. Three weeks later, Oberleutnant Friedrich-Wilhelm Marienfeld’s U-1228 attacked and sank the corvette Shawinigan in the Cabot Strait on 24 November. These two German attacks signalled the end of the Battle of the St Lawrence.

In May 1945, following Germany’s surrender, Kapitänleutnant Friedrich Braeucker’s U-889 and Oberleutnant Hans-Erwin Reith’s U-190 surrendered to the Royal Canadian Navy at Shelburne, Nova Scotia, and Bay Bulls, Newfoundland, respectively.