I played this recently and had the same issue. Ran it by the table and no one understood it. I ended up just making up rules that gave the players a bunch of fish so that they had a bunch of pips to spend on new stuff, since none of them seemed to enjoy the idea of fishing repeatedly for pips anyway.
I had them roll a d6 to determine the type of fish they found in a 'school' of fish (treating it as a d3) then 3d6 to dermine the amount of fish. Then I set a rating they had to beat based on how hard the amount and size of fish sounded. Then if they failed the next roll I let another person on the boat try to help them pull it in.
Players didn't seem to hate it. But did not seem to like that they rolled so much to fish. Again I think fishing was their least favorite part of the fishing adventure. They mich preferred mapping the river basin and uncovering all the secrets in it.