Almost all fish species in the Styx River catchment spend some time at sea. The exceptions are the upland bully, which do not go to sea at all, and the brown trout that migrates within the river, only occasionally going to sea.

Generally spring is the time when most species are migrating in from the sea to the river, and autumn they are migrating back out sea.

In terms of life history smelt and lamprey differ from the other species as they spend most of their life at sea and enter freshwater mainly to breed (anadromous fish). The remaining fish spend most of their lives in freshwater with adults moving into, or at least, toward the sea to spawn (catadromous fish).