They're slightly rare, I think. Sharks and the coelacanth are a bit tougher to fish up though in my opinion, and the coelacanth only appears during rain or snow. I believe the char appears near the waterfall but I could be wrong, I'm not completely certain. The trevally is about the same size as a tuna I think.
When your fishing you should try closing your eyes and listening for the sound of the bobber going down, that helps me get them all the time if you're stuck on reeling them in. Or if you're having a hard time spawning them, going into buildings or saving and continuing will recycle the fish available. Scaring away smaller shadows can help too!
I've seen char generally under or near the waterfalls.I think the Wiki say that they're "uncommon".I've caught the giant trevally mostly on the island.They seem to be pretty rare.
I think the char is one of the rarer fish to find in the game since they only spawn in a very small area. Giant trevallys always seemed fairly common to me.
The char only shows up at 4am to 9am or 4pm to 9pm.
I've found them at the bottom of the river a lot. You can also find them swimming at the top of the river near the waterfall.
Caught a lot there. The fishing tournament should be a good day to catch one.