Some people told me that they only take the place of items, so you catch an event fish instead of a bottle, key, relic, plump coelacanths, etc.. But personally I've caught quite a few non-event items thus far today. I realize that could very well be RNG, but it's enough for me to have doubts. When I asked them how they know it works the way they say, they just said they know from experience. 

My main reason for wanting to know, is because if the event fish take the place of regular fish, then it probably means I am making less money, because they can't be traded like regular fish can (and could even mean less red fish, depending on how it works?).

One thing I have noticed, after using a triple rod, is that I can catch a fish + an event fish in a single catch.

Can you normally catch a fish + an item in a single catch? I thought it was only a fish + another fish, or an item + another item. But I never really paid attention enough to be able to say this was the case for certain.

If you normally can only catch a fish + fish (or up to 3 fish) in a single catch, then this is pretty difinitive proof that the event fish do in fact potentially get caught instead of fish, and not just items.

If no one knows the answer to this, then perhaps if you can pay attention when using a triple rod, and let me know if you catch a fish + item (Not counting event fish.).

Event items are an additional roll on a regular catch.