In the case of the hydrographic units, determined by a large-scale longitudinal division, the large units are often less rich and the small ones often more rich than would be predicted by the regional species-area curve. The large units in the freshwater estuary, from downstream of Lake Saint-Pierre to the brackish-water estuary, are very poor in species. Similarly, the richness of two riverine lakes, Lake St. Pierre and Lake St. Francis, is low compared with that of the Lake of Two Mountains and particularly that of Lake Saint-Louis. Several small units in the Montreal Archipelago, like the Lachine Rapids, Rapide des Cèdres and the Rivière des Mille Îles, are rich in comparison to their size.

The dendrogram below summarizes the structure of the freshwater ichthyofauana in terms of the hydrographic units (the observed species richness is shown in parentheses). The most significant division is between the brackish-water estuary, of low richness, and the hydrographic units further upstream, which are much richer. Among the freshwater units, a second division isolates the freshwater estuary, poorer in species, from the units further upstream. The unit immediately downstream from Lake Saint-Pierre, which extends from Port-Saint-François to Grondines, is differentiated from the rest of the freshwater estuary group, more closely resembling the Rivière des Prairies in its species composition. The last main division is between units that are predominantly lotic (Rapide des Cèdres, Lachine Rapids, and to a certain extent, the Laprairie Basin) and units that are predominantly lentic (the four riverine lakes, the rivers in the Montreal Archipelago and the rest of the river).

According to the list of species favouring or avoiding the different environments, 41 species favour nontidal freshwater units that are lentic in nature, while 17 species avoid these units. Nine species favour nontidal freshwater units that are lotic in nature, while 24 species avoid these. Similarly, the freshwater estuary is frequented by 17 species and avoided by 13, and the brackish-water estuary is favoured by 13 species and avoided by two species. However, readers should note that species that avoid the estuary completely are not taken into account in this analysis.

Species that prefer one environment to another were defined according to their ecological characteristics as described in the literature and synthesized by variables such as salinity tolerance and affinity for various macrohabitats. Species favouring the nontidal upstream portion of the river tend to be stenohaline (intolerant of salinity). Among these species, those favouring lotic environments often have an affinity for streams. Species favouring lentic environments are often found in lakes and are the most strongly associated with vegetation. Conversely, species that favour the freshwater and brackish-water estuary are more euryhaline. Mobile species not associated with vegetation often have a strong affinity for river habitats rather than stream habitats.

Jacques Leclerc