According to this question freshwater fish should not be used in Ceviche--why not?
You should never eat freshwater fish in raw preparations. Freshwater fish are far more likely to have nasty parasites such as the lung fluke, that can only be killed by cooking. There is a slew of other nasty beasts that can be harmful if not killed.
Unless you want to end up on an episode of Monsters Inside Me, stay away.
In addition to hobodaves fine answer, freshwater fish usually contains a lot more mercury and other very unhelathy substances caused by human pollution. Some doctors recommend you to only eat freshwater fish twice a month.
Salmon is a fresh water fish that is regularly used for sushi, however it needs to go through extra preparation. Typically it needs to be frozen at -31F for seven days to kill whatever parasites are in there.
Freshwater fish as long as they are caught in clean water are totally fine to eat raw. Granted they may not be as tasty or have the mouthfeel of a fatty saltwater fish they are perfectly fine to eat.
To be honest I would much rather eat a wild fish caught in a clean lake over any farm raised 'ocean' fish that is fed garbage and has no nutrients.
If the freshwater fish you catch comes from a clean body of water and you want to eat it raw go for it. You will not get sick and will benefit from the high level of protein.