This class includes the largest number of living species of all classes of vertebrates, more than 28,000 species and accounts for about 96% of all fish species.
The species of fish found in and around the Outer Hebrides are predominantly marine.
Only euryhaline fishes were able to recolonize Outer Hebridean fresh waters after the last glaciation so that inland waters now contain only six indigenous, truly freshwater species; salmon, trout, charr, three- and ten-spined stickleback and the freshwater eel. Parts of some lochs are invaded by the sea at stages in the tidal cycle and some 20 species of marine fish may be found at times in inland waters. Outer Hebridean lochs in general have special scientific value because, unlike many Scottish mainland lochs, they have not been affected by the introduction of alien species and some are still in an unexploited state (Campbell & Williamson, 1979)