A few days ago I was griping about this exact thing. It drives me nuts that I need to do two fishing trips to complete the Activity. It takes longer than any other activity by a lot. I want to support a suggestion to lower the number of fish required for the Activity, but…
Then I got to the level 12 and unlocked Backwoods Brook. The fish jump faster there, resulting in more catches per trip. I went from 6-8 fish per trip to always 10 or more. I'm still doing 2 fishing trips, but it does suggest that the amount of fish you can catch per trip increases as you get to higher level zones.
The next zone unlocks at level 35, which is quite a hike from 12. But there's the Fishing: Winter Stream encounter in Icerun 2 that unlocks at Fishing level 20 that gives big fish rewards, so it's a good half way point to strive for. At the level 35 zone, can 15 fish be caught in a single trip? I have a feeling it might be. I'm only level 14 right now, I can't confirm, but it seems reasonable if the trend continues.
While having to do 2 trips to complete the Activity currently is annoying, it also makes me do more trips when I otherwise might not. I'm also getting more exp per Activity than it calls for - Exp is granted per fish, not per trip, so exp gain continues after the 15 fish required have been reached provided you don't navigate away. I don't enjoy the Fishing game so I appreciate the aid in leveling that's being asked of me.