If I go out and kill a newborn fawn in the woods for shits and giggles without the appropriate tags, out of season, etc. it’s still poaching, just the same as if I went out and killed an 8 point trophy buck I didn’t have a tag for, took it home, ate it, mounted it’s head on my wall, etc. That fawn may not have survived, it may not have grown into anything impressive, but at the end of the day I killed a deer I was not legally allowed to kill. The guy writing the law probably didn’t have killing fawns for fun in mind, they probably pictured something more like the second example I gave, but I think most of us would agree that the fawn-killer should be punished just as or maybe even more harshly that the buck-killer.
I can’t think of any good reason it shouldn’t be the same for fish.
EDIT: also, usually with fishing regulations, there’s also size limits, you can’t keep a fish under a certain size, it has to be thrown back. These fish were almost certainly under the legal size. Not to mention creel limits, even if they were somehow all of a legal size, and even if he somehow did everything else legally (which he didn’t,) I suspect the creel limit on salmon is significantly lower than 18,000