I never used it before but isn't it linked to task markers.
In such a case to set min_performers="10" I would think that you would need 2 fishing markers as they only allow 5 workers each.
Let me know if this is right or wrong.
One more thing, the game mechanism has limitations. Events do not initiate in some cases without any apparent reason, an event position inside the xml file seems sometimes to make a difference.
I am working on a new scenario and it is taking me an awful lot of time to get all the events to work properly. sometimes they initiate other times they do not.

Any way, good luck with your project.