Female largetooth sawfish give birth in estuaries, the pups already carrying a fully formed rostrum sheathed at birth. The young then move upstream into fresh water and remain there through their early growth, in some rivers for four or five years, before working back downstream as they mature.
The habit is genuinely riverine rather than incidental. Historic records place the species far up the Amazon, in Lake Nicaragua, and hundreds of kilometres up rivers in northern Australia and West Africa; nineteenth-century naturalists at Lake Nicaragua treated it as a resident freshwater fish.
That life history is also the trap. A nursery that depends on wet-season flow is undone by weirs, barrages, water abstraction and gillnetting in shallow river channels. Northern Australia now holds the most substantial remaining population, and elsewhere the species has been lost from most of the rivers that once held it.