A cluster analysis of depths produced a significant dendrogram, confirming the above-mentioned gradient. Deep waters (15 m and over) are differentiated first from shallower waters, because they are less rich. The shallower water group is then successively divided into four different clusters, which become increasingly similar at decreasing depths. A habitat selection analysis was carried out by comparing extensive shallow zones with all the deeper zones, retaining only five depth classes (0-1 m, 1-2 m, 2-3 m, 3-4 m and 4 m and over). This method is justified by the significant dichotomy between shallow littoral habitats and deeper channel habitats (Schlosser 1991; Bain 1995; Bain and Knight 1996).

The analysis showed that 52 species preferred the shallowest depth class (0-1 m), while eight species avoided it. Few species frequented the deeper areas, while many species avoided them.

The 0-1 m depth class was preferred by species that are small, relatively sedentary and spawn at an early age after moving only short distances to the spawning grounds. Deeper classes were preferred by species that can reach a large size, are relatively mobile and spawn at a later age after a longer spawning migration.

Jacques Leclerc