Hi! Hope you're doing great! Thanks so much for the comment and sorry for the slow response!
Catfish is the big one, as described on page 16. I feel maybe we should have added the names of the species in the diagram for clarity, your confusion makes total sense.
Since Fishing counts as an Action, it is up to the GM to determine difficulty like any other. It's a super rules light game, so the GM could simply define the Difficulty for a Fishing Action according to the fiction they're envisioning. Fishing near a waterfall could be way harder than fishing in a calm lake, for example. (page 9, The Catch Roll)
And, finally, the only hard limit on fishing is the equipment's HP, representing wear and tear to the fishing gear, although miss on the die drop table does not count as a failure. You need to hook up fish and fail the Action roll in order to damage the equipment. That's as far as limits go, rules as written, but of course you could always make rulings of your own in order to limit fishing activities
I hope this answers your questions! Feel free to get in contact if you need anything!
Cheers!