Battle of the St . Lawrence The Battle of the St . Lawrence involved marine and anti - submarine actions throughout the lower $ 3 and the entire Gulf of Saint Lawrence , Strait of Belle Isle and Cabot Strait from May – October 1942 , September 1943 , and again in October – November 1944 . During this time , German U - boat s sank several merchant ships and three Canadian warships , without loss . In the inter - war years , poor economic conditions and a sense of security , engendered by the proximity of the United States and the traditional protection of the British Royal Navy , had resulted in the Royal Canadian Navy ( RCN ) being equipped with very few ships , especially for coastal defence . Upgraded to six destroyers just before the war , Canadian naval deployment gave priority to the North Atlantic convoy routes and responsibility which grew until the war ' s end when the RCN was the third largest allied naval power in the world , with 100 , 000 men and women and 400 vessels . Historical context . From the start of the war in 1939 until VE Day , several of Canada ' s Atlantic coast ports became important to the resupply effort for the United Kingdom and later for the Allied land offensive on the Western Front . Halifax and Sydney , Nova Scotia became the primary convoy assembly ports , with Halifax being assigned the fast or priority convoys ( largely troops and essential material ) with the more modern merchant ships , while Sydney was given slow convoys which conveyed bulkier material on older and more vulnerable merchant ships . Both ports were heavily fortified with shore radar emplacements , searchlight batteries , and extensive coastal artillery stations all manned by RCN and Canadian Army regular and reserve personnel . Military intelligence agents enforced strict blackouts throughout the areas and anti - torpedo nets were in place at the harbour entrances . Even though no landings of German personnel took place near these ports , there were frequent attacks by U - boats on convoys departing for Europe . Less extensively used , but no less important , was the port of Saint John which also saw matériel funnelled through the port , largely after the United States entered the war in December 1941 . The Canadian Pacific Railway mainline from central Canada ( which crossed the state of Maine ) could be used to transport in aid of the war effort . Although not crippling to the Canadian war effort , given the country ' s rail network to the east coast ports , but possibly more destructive to the morale of the Canadian public , was the Battle of the St . Lawrence , when U - boats began to attack domestic coastal shipping along Canada ' s east coast in the St . Lawrence River and Gulf of St . Lawrence from early 1942 through to the end of the shipping season in late 1944 . Spring 1942 . The " Kriegsmarine " 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 ; therefore , early attacks in the Battle of the St . Lawrence were considered ad - hoc and opportunistic . The first attack was by , which torpedoed and sank the British freighter " Nicoya " at the mouth of the St . Lawrence River several kilometres off Anticosti Island on 12 May 1942 , followed by the Dutch freighter " Leto " in the same vicinity several hours later . " U - 553 " departed the Gulf of St . Lawrence to return to its established patrol in the North Atlantic . Before these sinkings , the Gulf of St . Lawrence and St . Lawrence River had been guarded by only four RCN warships , a , two Fairmile Marine motor launch es and an armed yacht ; a clearly inadequate force for the task . The RCN ' s response to the attacks was to deploy five s , but it remained inadequate even with these reinforcements . The incident revealed that the RCN did not have the resources to deal with the situation and there were political repercussions in Canada with suggestions that RCN ships allocated to the Atlantic convoys should be recalled to protect Canadian territorial waters ; however , the RCN ' s priority remained with the protection of convoys to Britain , the Soviet Union and North Africa . Several RN escorts were attached to the RCN for some months during 1942 , with convoys in the St . Lawrence River and Gulf of St . Lawrence being formed between RCN facilities at in Quebec City , in Gaspé , and in Sydney . RCAF aircraft carried out operational patrols from RCAF stations such as Mont - Joli , Bagotville , Chatham , Mount Pleasant , Charlottetown , Summerside , Debert , Stanley and Sydney as well as various civilian fields , particularly in the Magdalen Islands . Residents along the Gaspé coast and the St . Lawrence River and Gulf of St . Lawrence were startled at the sight of maritime warfare off their shores , with ships on fire and explosions rattling their communities , while bodies and debris floated ashore . The Canadian government ' s wartime secrecy saw censors forbid media reporting of incidents ; so the only news came from local gossip . Blackouts were strictly enforced and army units were sent out on coastal patrols along roads and railway lines . Summer 1942 . On 6 July , sank three freighters off the Gaspé coast and damaged another on 20 July , each time escaping attack by the " Bangor " - class minesweeper . There were further German attacks in August , on harbours in Labrador and Newfoundland and on a convoy in the Strait of Belle Isle that damaged the . In September , three U - boats made a joint raid on the St . Lawrence . sank nine ships and damaged another in a two - week period , escaping attacks by escort vessels each time and sinking the " Flower " - class corvette on 11 September . was less successful in attacking merchant shipping but it sank the armed yacht ; " U - 165 " was harassed by RCAF patrol aircraft and completed its operation in the Gulf of St . Lawrence with no further incidents . The continued attacks caused the St . Lawrence River and Gulf of St . Lawrence to be closed to all trans - Atlantic shipping , allowing only coastal trade . In practice , although this embargo strained the Canadian National Railway ( CNR ) system to Sydney and Halifax , it simplified the management of Atlantic convoys . The embargo lasted until early 1944 . Fall 1942 . In October , the Newfoundland Railway passenger ferry was torpedoed by , in the Cabot Strait , between Sydney , Nova Scotia and Channel - Port aux Basques , Newfoundland , with heavy loss of life . " U - 69 " escaped a counterattack by the " Bangor " - class minesweeper HMCS " Grandmère " . In November , sank two iron ore freighters and damaged another at Bell Island in Conception Bay , Newfoundland , " en route " to a patrol off the Gaspé Peninsula where , despite an attack by an RCAF patrol aircraft , it successfully landed a spy , Werner von Janowski at New Carlisle , Quebec ; he was captured at the New Carlisle railway station shortly after landing on the beach . U - boat losses experienced by the " Kriegsmarine " during 1942 following the entry of the United States Navy into the Battle of the Atlantic , coupled with declining 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 enemy submarines withdrawn from the St . Lawrence River and Gulf of St . Lawrence by the end of the 1942 . 1943 . Canadian military intelligence and the Royal Canadian Mounted Police ( RCMP ) intercepted mail addressed to several " Kriegsmarine " officers ( including Otto Kretschmer ) imprisoned at the Camp 30 prisoner of war camp at Bowmanville , Ontario in early 1943 . The correspondence detailed an escape plan in which the prisoners were to tunnel out of the camp and make their way ( using currency and false documents provided for them ) through eastern Ontario and across Quebec to the northeastern tip of New Brunswick off the Pointe de Maisonnette lighthouse where the escapees would be retrieved by a U - boat . Canadian authorities did not tip off the POWs and detected signs of tunnel digging at Camp 30 . All prisoners except one were arrested at the time of their escape attempt ; the sole inmate who managed to escape travelled all the way to Pointe de Maisonette undetected , likely travelling onboard Canadian National Railway passenger trains to the Bathurst area . This POW was apprehended by military police and RCMP on the beach in front of the lighthouse on the night of the planned U - boat extraction . The RCN provided a U - boat counteroffensive force ( codenamed " Operation Pointe Maisonnette " ) that was led by ( a Flower - class corvette ) , which was outfitted with an experimental version of diffuse lighting camouflage for the operation . The task force led by " Rimouski " waited in Caraquet Harbour , obscured by Caraquet Island , the night of 26 – 27 September 1943 and detected the presence of off Pointe de Maisonnette while shore authorities arrested the POW escapee . " U - 536 " managed to elude the RCN task force by diving just as the surface warships began attacking with depth charges ; the submarine was able to escape the Gulf of St . Lawrence without making the extraction . 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 reopened 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 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 " , a few kilometres off of Channel / Port aux Basque on the night of 24 — 25 November , with the loss of all 91 crew members , including former Toronto Maple Leafs hockey player Dudley ( Red ) Morine Garrett . Authorities only realized that it sank when the " Caribou ' s " replacement ferry , the SS " Burgeo " , sailed into North Sydney without the " Shawinigan " on 26 November , after it had tried numerous times to make contact by radiophone earlier that day . Wreckage was discovered on 27 November , and six - crewmen ' s bodies were recovered . It was the worst case of military deaths in Canadian territory during the war . These two German attacks marked the end of the Battle of the St . Lawrence . In May 1945 , following Germany ' s surrender , and 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 .