Sawfish
Sawfish are primarily found in coastal marine and estuarine brackish waters, but they are euryhaline (can adapt to various salinities) and also found in freshwater. The largetooth sawfish, alternatively called the freshwater sawfish, has the greatest affinity for freshwater. For example, it has been reported as far as up the Amazon River and in Lake Nicaragua, and its young spend the first years of their life in freshwater. In contrast, the smalltooth, green and dwarf sawfish typically avoid pure freshwater, but may occasionally move far up rivers, especially during periods when there is an increased salinity. There are reports of narrow sawfish seen far upriver, but these need confirmation and may involve misidentifications of other species of sawfish.
Largetooth sawfish
As suggested by the alternative name common sawfish, it was once plentiful, but has now declined drastically leading to it being considered a critically endangered species by the IUCN. The main threat is overfishing, but it also suffers from habitat loss. Both their fins (used in shark fin soup) and "saw" (as novelty items) are highly valuable, and the meat is used as food. Because of the "saw" they are particularly prone to becoming entangled in fishing nets. Historically sawfish were also persecuted for the oil in their liver.