I'll be honest, I haven't explored very far in the dungeon yet, however I'm pretty sure this isn't there... The fishing contest is one thing, but that can be pretty boring if you genuinely would like to catch and keep water pokemon. Seeing as we can't fish on normal maps, it would be interesting to see fishing added to the dungeon. Maybe even go the extra mile set up a fishing skill too where you can catch certain pokemon at certain levels (being able to catch multiple pokemon at certain levels would be even better for some variety) and let us keep what we catch. When I first tried fishing in creed, it was a giant disappointment to realize I can't keep anything I catch.

UPDATE: I realize I'd been a bit vague in my explanation, so to prevent any misunderstandings here's some extra information. There are more water pokemon (124 of them last time I check) than any other type which is why this could work so well.
In order to fish you could buy different tools apart from your standard fishing poles. Fishing nets of different sizes, cages and specialty poles for instance could provide you with a different set of pokemon to catch. Pokemon such as Krabby and Corpfish which are clearly meant to represent real world crustaceans could be caught using the same tools that are used to catch crabs and lobsters in the real world. Each tool could be used at a certain level. For example levels 1-15 you'd have access to a normal fishing pole. The normal fishing pole could allow you to catch fish looking pokemon such as magikarp, and feebas. At levels 15-30 you now have access to use a tool be it cage or net to catch pokemon that are associated with that tool, of course you can always choose to simply stick to the fishing pole.
On top of tools available at different levels, certain pokemon become avaible to catch at different levels. The higher your fising level, the more pokemon are available for you to catch with each tool. Say at levels 1-15 you can catch, Goldeen, Magikarp, feebas and/or a few others, then levels 15-30 you could still be able to catch the pokemon from your previous levels, however now you can also catch Psyduck, tentacool, or Luvdis, etc. If you catch a pokemon from a previous level it will give you less exp. All pokemon caught would of course be base Lv. 5 pokemon
If this is approved to be added into the game and who ever is in charge of adding it doesn't want to do all the foot work of assigning pokemon to levels/tools feel free to ask me to do that part and I'll do it. I'd write the code for it and just hand it in like homework if I could, but unfortunately I know next to nothing about coding, so lists and basic guidelines is all I'd be able to do.

it would lead to a flush of pokemons with a trainer and those will be free so it is 59% not good and 41%nice

It wouldn't be free... You'd have to buy a new types of fishing tools, and there is of course the rise of the fish getting away. Besides the map pokemon are practically free, what's wrong with that?

Great idea, I myself would like that to be implemented. It would make the dungeon that tad bit more interesting and you could keep the Pokemon you catch. Rate 8.5/10