In 1931, census data was only readily available for Montreal's 1931 wards, which covered the city limits of that time. The Papineau ward experienced the greatest population loss, 74%. It experienced urban renewal projects, such as the Radio Canada headquarters, as well as smaller redevelopments. This was in addition to the effects of gentrification, decreasing household sizes, and displacement of residential uses by employment uses, being close to downtown.
However, the Saint-Jean-Baptiste Ward (-70%) also experienced major population loss despite appearing relatively intact. It is centered around Rachel and St Denis streets and was the densest ward in the city, with 31,579 residents and a density of 38,231 p/km2 (99,019 ppsm). It likely grew a bit denser still during the Great Depression. Population losses of 50% or more appear to have been typical for neighbourhoods that were built out by 1931. It should also be noted that even neighbourhoods that grew from 1931 to 2011 may have peaked in population in the 50s-70s and lost a significant amount of population since.
Here's a map showing how the city grew during the Great Depression (1931-1941). The wards were often combined because they did not always match up to 1941/1951 census tract boundaries. This period was known for having little new construction due to the economic conditions. However, the city was still experiencing population growth, albeit slower than before or after. As a result, much of the added population would have been expected crowd into existing dwellings, and as can be seen, the core areas all experienced population growth. In fact, the Papineau ward (+12%) was the 3rd fastest growing in the city. The peripheral wards of St Paul (+7%) and Mercer (+4%) appear to have experienced essentially no new development with population growth similar to the core, though they would experience significant growth soon after.
From 1941 to 1951, the central wards would lose population to return to approximately the 1931 levels. Meanwhile, Ahuntsic, Cote-des-Neiges-Notre-Dame-des-Graces and Mercier would experience explosive growth, with other peripheral neighbourhoods also growing relatively rapidly.
Population loss would continue in built out neighbourhoods after the 1951 census, and in many core neighbourhoods would occur at a significantly faster rate in the 60s and 70s than in the 40s. This map showing population change from 1951 to 2011 paints a similar picture to the map in the earlier blog post, though with slightly different boundaries reflective of the 1931 wards.