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From downstream to upstream, the brackish-water estuary, the maritime estuary (including the Saguenay Fjord) and lastly, the Gulf are the main maritime regions of the St. Lawrence system. These regions form a vast estuary system characterized mainly by a strong salinity gradient. The gradient is longitudinal in the brackish-water estuary but becomes transversal in the maritime estuary. In the entire estuary system, roughly 123 strictly marine species occur, as well as 16 diadromous species (which live part of their life cycle in fresh water), and 33 freshwater species (whose populations are mainly based in fresh water).
A comparison of the ichthyofauna of these regions shows that species richness increases from close to 48 species in the brackish-water estuary to roughly 83 species in the maritime estuary and Saguenay and up to 141 species in the Gulf. Freshwater and diadromous species dominate in the brackish-water estuary, but the number of freshwater species decreases in the maritime estuary and Gulf, while the number of diadromous species remains more or less constant. The number of marine species increases significantly from the brackish-water estuary to the Gulf.
At a finer scale, in the brackish-water estuary, a decrease in the freshwater component accompanies the increase in surface salinity. Most freshwater species do not occur at a salinity greater than 5 p. 1000. The number of diadromous species remains fairly constant along the salinity gradient, with any differences observed due to varying sampling efforts. Lastly, only 14 saltwater species regularly frequent waters with a salinity of 5-15 p. 1000, but this number increases to 24 in waters with salinity of 15-25 p. 1000. These results confirm the pattern of aquatic diversity generally observed in a brackish-water environment, in other words low diversity compared with the surrounding fresh- and saltwater areas.
The compilation of records in the maritime estuary again shows an increase in the richness of saltwater species and total richness as a function of salinity. In the case of diadromous species, however, a bimodal pattern of species richness emerges: species richness is low at 27-29 p. 1000 surface salinity (in the centre of the estuary), and high at 25-27 p. 1000 salinity (particularly along the south shore) and at 29-30 p. 1000 salinity (along the north shore of the maritime estuary). The same bimodal pattern occurs for freshwater species, with greater species richness along the north shore, which could be related to the greater number of major tributaries in this part of the maritime estuary. Diadromous species showed a clear trend towards concentrating near the shore, a pattern documented for salmon by Dutil (1988) and for several other species.
Jacques Leclerc
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