There are no such things as "trash fish", just a plethora of people who don't know how to properly prepare them. The State of Indiana says that carp, bowfin (dogfish) and gar are "trashfish". The reason they are designated as such is because they infringe upon or prey upon the habitat of those other species generally considered to be "game".
Carp, if taken from clean waters is excellent fare. Take a look in a good French Cook book for recipes. Gar, if you've visited the Southern States or live there, can't be kept on the fishmonger's ice tray because it sells as fast as it gets there. Dogfish is another story. I have heard that one needs to put it on ice immediately after catch to avoid the meat becoming mushy. Then they can be filleted and cooked. I believe it, I just don't know how.
I guess the upshot of it all is that there are very few animals on the earth that can't be eaten with relish if you know what you are doing. I never figured out how to cook Opposum properly although I had a stew once made by a Cajun woman that was delicious. I won't eat buzzards or rats. I've had just about everything else and enjoyed it. Don't turn your nose up if you haven't tried it and don't make all inclusive statements about anything. If you're just parroting something someone else told you, it may not hold water.