What I don't understand is why I catch 3+ kinds of fish at a single spot, in a single sitting. I thought it would be predetermined by the equipment I use, but it isn't.
IMO there should simply be fishing locations where you catch only one type of fish. And make a general "Fishing competence" for the equipment you use, which increases how fast you catch the fish. Low ql, low-tech stuff e.g. taproot and woodfish = 5% fishing competence, High ql high-tech stuff like silk thread and the gold lure = 95% fishing competence. And you get a chance to catch a fish every e.g. 10 seconds, your success rate depending on the previously mentioned fishing competence.
Of course, this may be way too simplified *shrug*
I've never ever caught a plaice or salmon. In our group we had around 8 of us trying to catch plaice. Nobody found any, once.