That's surprisingly easy. It's been done for chickens. All that was necessary was to put groups of chickens in cages, and keep only the cage groups that were the best egg-layers. After only a few generations, chicken-on-chicken violence was way down.

>docile salmon

Hmm… that made me think of domestication, do you think we could ever have salmon as pets? What’s their character like? More like dogs or cats?

If those salmon are too docile, just give them cocaine.

https://www.science.org/content/article/cocaine-pollution-gi...