i suspect that its because you will have two people quite close together for a long time.
I think the 'test and trace' rules only get applied if you have been in the company of someone for > 15 mins, otherwise they arent interested as its viewed that the longer you are near someone with the virus the more chance of getting it.
If your going to be out fishing for the day that is a long time to be in close proximity to someone else, so i would suspect that is where the rule is coming from. Although i may be wrong of course !