Post a log file. There's a sticky at the top of this forum detailing how.
Also, run a 6MB test at
http://testmy.net/upload to get a real-world view of your constant throughput. A basic list of system specs/brands doesn't really help.
Newer games tend to be less-optimized though. Have you tried restricting the game and OBS to separate/non-overlapping processor core affinities?
From the minimum system specs listed on Steam though, appears Fish Spirits requires 6GB RAM; I'd recommend keeping an eye on utilization while playing and casting, as you could potentially be running out with the additional demands placed on the system (and OS overhead).